Scarlet Heart was a CN drama,

Our main character had a near death accident, which send her back in time to Qing Dynasty, she gets into the body of one of her past lives as Ma'ertai Ruoxi. 

In 21 century she has a twin whom she does not know about and she is the mistress / girl friend of Yin Zhen but during Qing Dynasty her sister is the mistress of Yinsi (8th Prince)

We don't have S2 for Scarlet Heart Ryeo yet but if we have that snake princess as girlfriend / mistress, they will be getting all the thunders on them..  (screenwriters not the actors)

During Qing Dynasty she had close relationships with the fourth prince Yinzhen (Nicky Wu), 8th prince Yinsi (Kevin Cheng)  whom she was inlove before Yinzhen. Tenth prince Yin'e (Ye Zuxin) who falls in love with her, and fourteenth prince Yinti (Lin Gengxin). She also forges a close friendship with the thirteenth prince, Yinxiang (Yuan Hong).

Ruoxi first falls in love with Yinsi but after he chose the throne, she fells apart from him.

She fell in love with Yinzhen... but he also chose The Throne, despite that she stays with him, day by day Yinzhen gets more cruel not towards Ruoxi but towards other ppl and his brothers..

Her health gone worse, when Yinzhen orders her best friends to be killed, as she was the spy of the 9th brother.. 

She got really upset... 

Without knowing that she's pregnant Yinsi's wife tells her that he only acted against Yizhen because of her advice, Ruoxi gets into shock this leads to miscarriage & she could not convince again

Before Yinti (the sweet 14th prince) asked Ruoxi if she ever wants to leave the palace just say I accept and I will help you..

To help Ruoxi Yinti goes to Yinzhen and tells him that he wants to marry Ruoxi, but before that Yinsi tells Yinzhen that they promised each other to marry before him, and he disclose their past romance, which made Yinzhen angry and let Ruoxi go with Yinti, but despite Yinti's good care she dies, when Yinzhen learns about her health condition it was too late.

Ruoxi send letters to Yinzhen but since her calligraphy is too similar Yinzhen's, Yinti put the letters into another envelope, but jealousy hits Yinzhen he does not even read the letters until the last minute..   

With the death of Ruoxi, Zhang Xiao (herself in 21 century) she wakes up in the hospital learns that she was in coma for a couple of weeks, she wonders if what she experience was a dream or real, so she starts to research about the Qing Dynasty, but there is no record about Ruoxi, she decides to go to museum to see the artifacts from the Qing Dynasty, she falls into tears, & there she meets Yinzheng the stepson of Kang Zhentian... so she stares at him for a while and when YingZheng lefts she starts to cry this is the end of it.

6 years after Ruoxi's death Yinzhen dies.. 

In season 2 Yinzhen had a jade thumb ring just like Yinzhen in Qing Dynasty that was one of the things that reminded her the time spend in that era.. She could not keep herself secretly going to Yinzhen's office and having a close look. ( but mind you this was in S2)

How about the accident that causing Zhang Xiao to lose her memories? bring her back from Qing Dynasty when she was just about to solve the riddle make her lose her memories, make her fell in love with  Kang Sihan and bring her memories back with a sad event, try to show this as a good event by making Yinzhen understand that he loves Zhang Xiao not Lang Lang.  plus make everything go mental case after this incident. great this is bullying the audience..   (Scarlet Heart S2) 

I am not sure if there is another CN drama to help us to understand this drama better but there are few Korean Dramas which helps us to understand the drama better.. 

To be honest I loved the music on Scarlet Heart more comparing to the Korean Version...

Now before you jump to Korean Version listen to this.. Yinian Zhizhuo (一念执着; One Persistent Thought)"

So shall we jump to Scarlet Heart Ryeo

During a total eclipse of the sun, a 25-year-old 21st century woman, while she is trying to save a young child from drowing Go Ha-jin (IU), is transported back in time to Goryeo Dynasty. She wakes up in the year 941, in the body of 16-year-old Hae Soo and there, she encounters the many royal princes of the ruling family. She initially falls in love with the gentle and warm 8th Prince Wang Wook (Kang Ha-neul), and later Wang So (Lee Joon-gi), the fearsome 4th Prince who hides his face behind a mask and is given the derogatory label "wolf dog". Meanwhile, rivalry and politics ensue among the princes, in a fight for the throne, as Hae Soo finds herself unwittingly caught between it all.

While staying at Wang Wook's house,  at the time Wang Wook was married to her cousin, but there was no love in between Wook & his wife. His wife was in love with a army commander who got killed by her father, in order to make you her daughter to marry King's sons.  During this time Wook & Soo fell in love, and this love was fully supported by his wife..  

Lady Hae (Wook's Wife) gives Soo with a host of medicinal herbs courtesy of eighth prince Wook, Soo expressed her interest before, Soo was so happy and she made soaps for both Lady Hae and the queen. Wook watched that with a smile on his face.. ( this was around ep 5 or 6)

Lady Hae expressed her wish to find a good husband for Soo, but Wook's facial expression changed. 

Lady Hae asks Soo to go library and wait her there but she finds Wook instead of her cousin.. 

On the day that Soo was in the library she adored his calligraphy..

Wook decides to give her a poem instead, and instructs her to read it carefully. hence she can't read Hanja characters, she hopes her slave can read it for her.. ( silly girl how do you expect your slave to read the things you can not read )

But Baek understands that she's illiterate, so he starts to read for her, but at that moment Lady Hae arrives and finished the poem,  Baek understands that it's a love poem and murmurs if his bro out of his mind ( yup all your brothers lost their marbles for the love of Soo --- eheh this also includes you Baek)

Soo draws a reply to the poem 

After his wife's death, they secretly see each other,  but they could not get married as Wook chose the throne not her...

Soo falls in love with Wang So later on,  but he also chose the throne, this time Soo stays by the man she loves. 

After So sits on the throne, he became brutal.  He marries his half sister..  

He killed his own brother (10th Prince Wang Eun) & wife. 

He offered a title to Soo, saying once she gave birth to a son he'll give her a title.. 

Soo's health gone worse after he ordered her friend to be killed. 

WangS killed her as she was the agent for his brother 9th Prince..

WangS sends his 14 brother away from the Palace and before Wang J leaves the palace asks Soo if she ever wants to be out of the Palace just tell me I want..

After a certain time, Soo sends a message to Jung saying I want.  Jung comes to the palace telling Wook that he wants to marry Soo, just to take her away from the palace. 

just before that WW revealed their past romance to WS, he did not believe it and he asks this to HS, when she confirms it he says I do not ever want to see you again and that's it, the last time he saw HS properly was that day ( that's why we say be careful what you wish for)

After WJ & HS got married and moved away, WS secretly spied on them or send men to spy on the HS & WJ couple..  

One day while he was spying he heard that HS is pregnant, obviously he thought that HS is pregnant to WJ's baby.  let's the baby topic here and move to the next..

With full of anger he returns back to palace & says I do not want to hear about WJ or HS, stop watching them.. 

About 6 months later Hae Soo gave birth to a baby girl,  WJ tells the midwife that tell everyone that this child is stillborn, I guess he doesnt want Wook to find out, when Soo asked him to do a favour, Jungs asks if she wants to show the baby to Wook, she nodes, and she tells that she doesn't want her to grow up in the palace & asks Jung to take her of her like she's his. One last time she sends So a letter and she waits for him.. 

Despites of Wang Jungs good care Hae Soo dies soon after she gave birth to her daughter..

Wang S learns the death of his love  & the astronomer says but she send you many letters, he says what letters? Finally it bangs to him and finds the envelopes with Jung's calligraphy, hence Soo's & So's calligraphy is so similar he put all the letters into another envelope and send to WS

When WS reads the letters he rushes off to see HS but it was too late. 

I will leave the videos speak for me.. 

I seriously wonder how did they find this girl that looks like IU & Lee Joon Gi..  :) if this girl was their own daughter in real she wouldn't look like them this much..

After Soo's death Go Ha-jin wakes up in her own body, and learns that she was in coma for the last one year..

We know she is a MUA... so here is the back to the future video

SO let's compare

SH & SHR 

In Both dramas our main lady fell in love to prince 8 & 4

In Both the men they loved chose throne over love

Both had a bad health because of the men they loved..  

Both of their friend killed by the men they loved
Both are loved by 2 handsome princes but could never be able to tie the knot..  

Both could not have a happy ending
Both worked in laundry
The girls in laundry stole Rouxis hair pin..
Both girls were under the protection of Princes even when they were working in laundry
Both had got princes to shield themselves when it was raining
Wang So got hit by an arrow for Hae Soo
Court Lady gone under gallows for Hae Soo

Both could not see the men that they loved for the last time not even at their last breath...

One lived in Qing Dynasty 

One lived Goreya 

after the death of their previous selves they returned back to 21th century

Scarlet Heart Ryeo Soundtrack