Am I doing something wrong?

My boyfriend and I have been together for 3 years. We have been living together for almost 2 years. We went from having sex almost everyday to about once or twice a month. When I try to have sex with him, he says he’s tired or not in the mood. But I have caught him masturbating in the morning and even doing it in the bathroom at his work. I know he did it at work because I looked through his phone. I know that’s wrong, but it wasn’t behind his back. I looked at his internet history right in front of him. So I know he gets the urge to do something, but it’s not with me. It has been making me feel real bad about myself and I have expressed that to him. But nothing seems to change. I even lost weight to try and be more attractive to him. Is there something I’m doing wrong or am I needing to try harder?