Being in the closet is not psychologically healthy. It will mess you up over time.
It's also bad and weird that you guys don't have any alone time. Why not? Why wouldn't he create that space, if he really wanted you in his like in a serious way?
But worst of all is just the general lack of respect and love going around. If she was a loving person, she wouldn't treat you with such condescension (seriously, what is with that weird ownership-y, rubbing it in your face thing, wtf is this high school?). If he loved you as much as it seems like you love him, he wouldn't put up with her treating you like that. If you loved yourself as much as you love him, you wouldn't stand for being treated that way.
What I'm trying to say is, this is a bad situation all around. It sucks like hell to leave someone you love, but this is not healthy, and you have choices. It doesn't have to be this way. You'll get over him, I promise.
Some reading on how a situation like yours could actually be done healthily: