That awkward little mating dance we do

There are two very awkward stages of relationships.
The first is when you first meet, and have been through all the basic information but don't know each other well enough to start joking around or share personal information.
The second is when you've hit the stage when you're starting to commit. To me, this is like fumbling in the dark. I never quite know what's expected of me because I usually end things at this point as I start doubting the guy and/or find someone new.
With M though, I want to be with him, so I'll put up with him being my "sort-of boyfriend" for a while longer. It would just make the whole being faithful thing so much easier if I could tell guys I have a boyfriend so they know that I don't want anything more that friendship, because even if they seem innocent, I get the feeling that a lot of guys that I hang out with are sort of hoping for something to happen.
I don't know how to deal with this situation. It makes me crazy insecure, which makes me act all weird, which I notice about myself and that makes me even more insecure, so you can see how this is a spiral. I don't know how much I can depend on him, I don't know if I can call him and cry about my day and how I miss him, I'm still not convinced that he really does want to be with me. It's a sort of wobbling, like a toddler doing one of those puzzles where you press a shape through an opening and can't quite seem to find the right angle.
I guess I'm being immature about this. It's just a question of definition.
We have each other, that should be enough right?
The only purpose of making it official would be to keep other guys off me, which would definitely make it easier not to stray.