What do you do in a situation like this?

I'm in love with a boy who is in a relationship with someone else. When we first met he disliked the relationship he was in with that person and spent a lot of time developing our relationship because he was planning to brake up with that person. He still loved them though, so when that person found out about us they forced him to make a decision as to who he would be with and his decision was that he would try to work out the issues with the other person and spend more time getting to know me. We continued to talk about how our live together would be like in the future - because at that point I was waiting for him, waiting for their relationship to end so that we could be happy together. But over time I started realizing that they weren't going to break up. I tried to get away from him and move on, but he keeps me in. He tells me that he still loves me, but I am hurting. I am extremely jealous of the other person because they most all of his affection. I know they have mistreated him in the past and I don't want them to hurt him again. Our relationship went from being very flirty and loving to being more of a very very close friendship because of the other person...I can't wait for him forever like this, it's tearing me apart inside and is effecting me in a depressive way. But I love him so much that I'm willing to wait for him but I'm hurting so much...I just don't know what I should do.