She wanted to marry him because she felt herself falling out of love with him through their relationship but Gaeul focused on his dreams of being a singer. She had a hard time letting go of him because she was in love with him for years, even before dating. Gaeul is a free person and she wanted him to be hers. Now we see that he’s changed and trying to be better but Jiyoon decided to put their relationship behind her because she knows she’s not in love with him anymore, not like she used to be. She finds it hard when he always talks about going out again because she relies on her perfected memories rather than her true feelings.

I feel dump asking this but I didn't really understand why she broke up with Gaeul
Can someone explain?