Is it ok to wait for a person to love you back?

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I've known someone, she has had a tough past, she had a daughter with someone who told her "I am not going to marry you because i don't love you." Of course that made my friend grieve for so long. Until when she met another man, whom who love's her and her daughter so much, yet she still can't forget her past. She still love the person who has left her in the middle crisis. Why is it like that? why can't she forget the man who has hurt her so bad?