Solnyshko

She exposes the fake illnes of the other gurl and dumps him completely. He finds out she died that night on the road and makes a new deal: he rxchanges his life for her ability to love. In the end he dies but she doesn't love him but there is a possibility for her to love someone in the future

You missed the main point: the heart was never her beloved one's. She was dedicated to be there by his heart, endure anything just to be next to his heart and than she's told that the heart is not his. She went through so much pain only to know the one she loved was gone forever and even the heart she was living for was never his.
At the same time ML also learns the truth - she never loved him, and she cared about him only to keep that heart beat a bit longer. However he never even loved his so called white moonlight. He just got involved with her because she was FL's sister and he really liked FL back in school.
The title "you're not him" includes it all... even the heart is not his

Aslo, the whole story is told by her, how would she know things at his end if he didn't tell her later?
And one more little thing - the name of the drama - "She's not well behaved" - does assume a happy ending for FL.
If you watched The Double (marriage of di daughter), there too many people thought the ML died because there was no other option however if you read the original novel - ML never dies even close. I hoped this is also based on a novel but it seems no...
Don't get me wrong. I would prefer a clear unhappy ending, showing her living her life alone without him with sweet and bitter memories because in that case it would be an Oscar scenario, but vdramas are made typically for women over 30-35 who misses their youth and are longing for a romance, strong and complicated scenario is not something they would watch, so this scenario should have the happy ending. Especially considering the last words on the screen about them shining bright like stars... if we assume he died... well she also died in that case

Yes, BUT:
1. She was very surprised to see him. In delusional condition she would be just happy not surprised.
2. He just went for a gang fight, who says he wouldn't survive.
3. When he came up from the sea he didn't have the necklace, she was holding it, but later he had it on and she didn't hold it anymore
4. Their conversation about clear conscience: he said he was guilty...

He did not die as I think. At least it's not obvious

It was her role. Her character is an innocent girl who had never had any relationships because her "uncle" pursued her all the way and was driving out all the gys around her. Even when he tried to seduce her, nothing happened. When they were kissing ML even thought - she has no experience.