Why do I feel this way about my boyfreind?

We've been together for almost 3 years but have also been a part for a lot of the time. The most recent time he's been gone for was 9 month's. When he's not with me I call him and wonder when he's coming home but then when he's home it's almost like I want him gone. Our trust between the 2 of us is sh** cuz he's cheated on me and he thinks I'm gonna go retaliate and cheat on him. I would never do that, but what do I do???