I understand you are angry but ultimatums and trying to veto a person is not fair to others. You've put him in a position of making a painful decision over two women he cares about (loves?). I don't think you look like a shining light of being the better option in that because you've forced him to decide.
Your only option as I see it is to decide for yourself what you will do. Leave or don't, its really up to you, not either of them. You only have control over your self. Not over them or this situation, even if you think you do right now.
If I were in your shoes (having learned from my own experiences in this), I would let him know that you have certain requirements that need addressing and need to come to some agreement about. I would ask that the two of them and you sort out some boundaries around what she requires and he requires as well as you. If she is unable to do that, is in competition with you or things continue to stay the same even though you've talked about it then I would be telling him that I have no choice but to end the relationship as a result and that that is what it will come to. I would give it as much time as I can to see that change is coming and that they are adjusting. If I see they are trying I might be more patient. Things take time. If I see no movement and continued resistance, I would say goodbye, knowing that I am walking away having been the one that is most considerate, respectful and caring.
No one wins in ultimatums and vetoes. Everyone leaves feeling like crap because the one who gets the partner in the end will always feel yucky about how they got there. The one who leaves will always feel yucky that they were treated badly, and the shared partner will always feel they had no control and has lost a love. They generally don't get any support for their grief and that generally means a rift in the relationship that's left. That's what I think any way.
Do what is right FOR YOU, control yourself, don't attempt to control them. Ask that they do the same. That's all I can suggest.
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