I am tempted to think that your constant second guessing of yourself and thinking your "crush" always was thinking about and wanting to be with other guys probably shone through clearly, and thus reinforced those feelings that may not have been there in the first place.
I also don't think you should think of her as a "crush" when you've entered into a relationship with her! Crush means a girl you furtively admire and hope to be with. You were already with her.
It also sounds like you should be putting forth more effort on all levels: relationship, family, work etc. I know this can be hard to do, we've all been depressed, and it's very immobilizing.
Recently, I have realized that I personally need more structure. Setting big goals and objectives on my horizon (i.e. travel, new classes) made me really get my act together. In having these, I have been more motivated/productive with work as well.
I would advise you to undertake a new hobby, like a challenging new sport or musical instrument...something that can both give you a little more excitement in the morning when you wake up in the morning to look forward to and could potentially be a medium for meeting and bonding with new people.
As far as your family goes, it sounds like there are certainly some deep rooted issues to be dealt with, and it's FANTASTIC that you're in therapy; don't give that up, and make sure you're open and honest in your sessions. It might really help to move out, if you can afford it. If not, continue to strive for higher standards than they have for themselves, as they are obviously not sufficient for you.
I can relate to you also on a matter of being "too serious". My friends often tease me and tell me I'm an "old man", but I prefer to think that I'm just slightly ahead of the curve. I'm thinking about things more complex than unsatisfying sex (of which I've had very little) and drinking and drugs, I'm over it. I'm more interested in satisfying relationships, and at such a young age (I'm 21 as well) really getting to know myself, what I really like and value, etc. You have to remember that we're essentially toddlers in the realm of adulthood.
Persist in your efforts and never give up hope or lose faith in the fact that things will improve. Everyone experiences ups and downs, but it's how you handle them both that really counts. I hope this helps.