When Mingus was a stallion, he suffered from testerone poisoning...then we had him gelded. For the first few weeks he was even worse, which I wouldn't have thought possible. Then he settled into his "boss gelding" persona -- aka the boss of everything. He would "ride" Thelonius and Flash and boss them around. Around us he was a big love goober, but he tried to strong-arm us -- and as a big B-sized mini, he was very powerful.
The answer with him was giving him a job: driving. Part of his driving training was installing power brakes -- an absolute whoa. He is still a proud, spirited horse, but he is beyond trustworthy between the shafts. I can trust him with anyone -- to the point that he plays babysitter with anyone he doesn't know (at times to my embarrassment, as Al B will confirm).
Another important thing is to reconcile your innate personality with being the "boss mare." Keith and I are both softies; our wonderful trainer/coach pointed out that we didn't need to be harsh or break his spirit -- we just needed to be firm, consistent and follow through on whatever we asked.
These days, Mingus is the boss of everything except Keith and me. He still likes to test, but each time the answer is the same -- do what I say and we'll all be happy. Grab for grass while in halter? Let's not and say we did. If he is a good boy, he will be rewarded, but NOT UNTIL IT IS MY IDEA. We aim to always present the same message: This is not a democracy, but a benign dictatorship.
This is my verbose way of saying that a dominant personality will not necessarily be altered with gelding -- it will simply not be hormonally driven. I would approach him as I would a new horse, starting from scratch with training and getting the help of a trainer if needed. Remember that every moment you spend with your horse, you are teaching them. Don't give up on him...it may be a long road, but it's well worth the time.