Thanks for sharing your thoughts and perspective. The damaging thing about boxes is that they tell us who we should be and what we should want but that they keep us from really understanding what we do actually want.
As someone who has played a little in the BDSM world I want to tell you that being the top or the dom takes a lot of work and energy. It can be fun, but you are the one kind of holding the energy of the scene. The bottom/sub is free. Since the partners (if done correctly and not abusively) have negotiated the terms of the scene beforehand, she (or he) will only experience things that have been agreed to. It’s hard to describe just how relaxing that is, to have to make no decisions, and do nothing but exactly what you are told to by someone who you trust. Pro doms bear this out, that typically their clients are men with very demanding jobs, who are looking for a respite from that. There may be some element of this dynamic in what you are describing — the world tells you that as a male, you should always be in control and in charge, but a part of you actually desires a break from that.
I’d also encourage you to work with the insecurity piece, not just for the personal benefit of owning all of who you are and letting that be OK, but for the way it will impact your relationship. If your girlfriend has to hold back on who she actually is in order for you to be more comfortable, then you aren’t being fair to her. And, if you are able to allow her to be her full self, emotionally, powerfully, sexually — you are the one who will reap the benefit. An empowered woman who doesn’t have to censor herself for her man is a fun woman to be in bed with! I’ve written about that topic quite a bit also. The Anatomy of a Threeway articles have quite a bit about that if I’m remembering correctly.