Thank you all for your help, I know you guys are all right, but I guess I was hoping for a different answer.
The more I go without it, the more I want it. And I would feel terrible cheating because it's not like it's his fault that I need something more to be satisfied, it's my thing. I just feel like I wouldn't be able to forgive myself if I only left him because of that. But on the other hand, I know how happy I would be if I could somehow have that in my life. And with my luck even if I found a compatible dominant, the rest of our relationship wouldn't be able to compare to my current one. I guess I'm just scared. I just wish I would have thought about this four years ago.
Thank you for your advice Thir, I'll have to try and find a way to bring up the idea of a poly relationship. Although it seems like recently whenever I bring up things that might make our relationship better he freaks out because he's worried I'm trying to push him into something long term.