Any other mono secondaries? How do you handle your relationships?

I feel like I'm in such a strange position. I live with my boyfriend and his wife. They've been married 40 years, he and I have been together 2 1/2 years. I honestly don't know if I can handle this much longer.

For starters, I am a secret, so to all their friends and family I am "a friend who needed some help getting back on her feet". None of my friends our family know I live with them because anyone who knows me would never believe that story.

I have a bedroom of my own and they share a room. Every other night he sleeps with me. I'm starting to get to the point that I absolutely hate sleeping here by myself. If I'm going to sleep alone I'd rather sleep at my house, which I've started doing. This had helped some with my frustration, but not completely.

I'm mono not by choice, but because he doesn't think he can handle me being with anyone else. The problem is, always being second is really starting to take a toll on me emotionally. Sometimes we talk about me dating, but he's unsure if he would want to stay with me. Then I tell him I'm going to try and hang in a little longer. He's says if I'm going to leave him he wants me to do it sooner than later before our feelings get even stronger, when it will hurt even more to break up. I just don't know what to do. How do other's in this position handle being secondary?