Encourage her to go to therapy. If she won't--because she doesn't think she has a problem--then I'd consider walking. But I'd give her the chance to fix it first, since you care about her.

A lot of pp's projected that this gets worse with age, but I actually think that this is a trait that can easily lessen with age - a lot of women get more confident into their 30s and 40s.

Dump her.

A friend of mine was in a situation like this. We talked about it extensively a few times and I kind of figured he'd break up with her. They ended up getting married and having kids. I like her, so I'm happy for them, but I know the insecurity is still an issue and he's asked me a couple of times over the years if I can reassure her on this matter or that.

Most of the most insecure women I know are married, so I have to assume that most guys just don't care about that. I would find it difficult, personally.

You do know that no amount of validation from you is going to make her feel better and more secure. She really has to go to therapy and work on herself. You shouldn't have to be her therapist. I'm sorry, it's so difficult being in a relationship with someone like this.