I keep dreaming about my boyfriend's ex...?

I'm not quite sure why you're obsessing about her. To be honest, it's almost like you have a bit of a crush on here. As awful as that probably sounds to you, it's not that out of the ordinary. There could be attributes that she has that you want to adopt to keep your boyfriend close, which could behind a bit of a crush. Anyway, I don't really think there's any cause to worry here. He has given you no indication that he has any intention of leaving you for her. He may think about her occasionally, but that's only natural given their relationship. As you get older, you will find this more and more no matter whom you date. Everyone is going to have a history and have some baggage, and you just need to learn to handle it and not let it derail your relationship. Every guy will have some girl who got away, some ex he still thinks of fondly, etc...you just have to trust the person you're with and have confidence in yourself.