Does anyone get nostalgic for an old relationship?

Hi, I’m new here so I thought I’d just jump in. I have a friend who’s recently confided in me that her relationships aren’t working because she’s still in love with her ex girlfriend that treated her like crap. She’s met other people that are wonderful and she’d be lucky to have them in her life. When I’ve had a relationship end, it’s over so I’m having a hard time understanding her plight. Any one else that can’t get over their past?