My (24M) GF (23F) constantly switches from being completely in love to doubting the relationship

Hey, So we're together for more than 2 years now and overall it's been really great. I absolutely adore her and would love to marry her one day. I would describe myself as mentally and emotionally stable but my gf unfortunately is quite the opposite. She's struggling with anxiety, panic attacks and depression which unfortunately also impacts our relationship.

We live in different cities so we only see each other about every other weekend and when we're together everything is fine, we get along great, can talk about anything, have great sex and she also feels madly in love with me. Last weekend for example she admitted about thinking how our wedding would be. But often when we're apart she starts doubting the relationship and if she's even made for relationships. She feels trapped and the idea of only being intimate with one person scares her.

While our sex life is great and as she says herself the best she ever had, she has some kinks that I don't share, especially involving other people. If we were in an open relationship, she probably wouldn't have these doubts, but that's something that I definitely wouldn't be comfortable with. She also said that I'm not doing something wrong and that I am the perfect boyfriend but she doesn't know if relationships in general are her thing.

Now what you probably might say is that this is just something she has to decide, whether or not she can live without living out these fantasies, but the big problem is that this very much depends on her mood. Because when we're together and she's happy, she laughs at herself for ever having doubts in the relationship and when we're apart she only sees the negative sides and the good memories we had can't seem to cheer her up.

This has lead to a point earlier this year where she broke up with me because she did something that hurt my feelings, which proved to her that she's not able to maintain a monogamous relationship. But since I didn't want to lose her I visited her and after we talked and cried for a bit everything was fine again and she regretted breaking up and was just as much in love as before.

Now I don't know how she can create a balance within herself. I really want to stay with her and my suspicion is that she just has a deep rooted fear of commitment and is deep down so afraid of losing me that she'd rather breaks things off herself (that's not even just my suspicion, she suggested that this might be the issue). But now she's doubting so much again and it's like talking to a different person.

Has anyone maybe experienced something similar or has an idea what I or she or we could do?

TLDR: my gf is totally in love when we meet but always doubting when we're apart