Never Ending Cycle with Bipolar Boyfriend

I've been in a relationship with my boyfriend for 7 years. We have problems every time around the same time, usually during his manic phase. He was diagnosed 4 years ago during a time we weren't together for a year. Before that time I always knew something just wasn't right with him. His mood would change frequently. I couldn't understand how we could be so in love and every year around the same time he wanted to be single. A year after we got back together he relunctly decided to seek treatment. Within a year he stopped and the cycle began agian. He got back on medication and now he's off again. When he's manic he says and does hurtful things. He has cheated on me in the past as well. I love him but I don't know what to do anymore. When I look at him or hear his voice it's like it's a different person, it's like he's empty. He told me he doesn't want to take medication anymore and he knows it may be wrong but he's willing to take that chance. Has anyone gone through this? Is my relationship just doomed? I try and work with his moods but I feel like all I do is worry about him in the relationship and my feelings are often pushed to the back of my mind. Every time I try to tell him he's doing things that aren't good for the relationship or something that has hurt me, he says I'm complaining. I try really hard to let some things go but I get so frustrated how he treat me so poorly. Half the year he's truly dedicated and I can see a long future with him but times like this I don't even know why I stay. I'm getting older and I'm wondering if this is what I want for my life and that it will never change. I want to make this work but I'm giving him one more chance to realize he doesn't have a handle over his mood right now. I've even asked him if there was any way I can talk to him differently so we can make this work. Does anyone have any suggestions or advice? I don't really have anyone to talk to. Not every day you meet someone who is in a relationship with someone who has a mood disorder. Since I've been with him I now suffer from depression and anxiety over this whole situation. It has literally caused me throw up at the thought of it. I know this isn't healthy. Just for more information, some things I bring to his attention is us spending more time with each other, him forgetting we had plans, his attitude when he talks to me, etc...I feel like it's my fault and he constantly blames me. I know it's not all my fault and he knows I've taken him leaving ever year personal even though he has said it had to do with his mind state. It's just so hard to have someone you love fall out of love with you over and over again. He once described it as losing him to the darkness and I couldn't pick a better description. I wish I didn't have to lose him. Any insist or opinion is greatly appreciated.