Hi! Want to say first that this was something I haven't read. I only saw this on a different website and it piqued my interest but no one's replying to OP. I'm going to quote OP:

"Hi guys! Can someone tell me the title of the manhwa I suddenly remembered reading? FL is a regressor/reincarnator who has a book or system window. In the first round she successfully finished the events to help ogFL & ogML rescue the world by sacrificing herself (she needs to die or the story won’t move forward). I think she went to heaven?

But suddenly, she went back in time. But her system window is now bugged with no memory of the story. Then later it’s shown that ogML turned back time to save her because he’s always been in love with her. This time he has her book and now has his own system window.

If anyone knows the title please let me know! Thank you!"

Hope someone can identify it. Thank you!