So the first thing to say is that depression can make us feel detached from the people close to us, and also like we're not good enough for them. As someone who's recently entered into a new relationship, this is something i also experience. It'd be well worth speaking to you psychologist about this.

Do you mind if i ask how long you and your girlfriend have been together? does she know about your depression? I'm not going to suggest that you necessarily should or shouldn't talk about it with her, but it might help if she can understand what you're going through.

As for thinking/dreaming about other people, it's perfectly natural, and as others have said, it's perfectly fine as long as you don't act on it. I'm fortunate that my boyfriend and I can check out guys together (im guessing this is less of an option for you) but neither of us want an open relationship and we're respectful of that. we all find a range of body types and facial features etc attractive, and that doesn't stop being the case just because we find a partner.