The solution is to make plans to move out as you begin to narrate the discomfort with these situations. Living with your mom and her boyfriend is something that you must see as temporary. Even if leaving is years down the road, you need to have a plan to move on and be working toward it. The boyfriend likely sees you as competition for attention and a distraction for his girlfriend. While you didn’t mention the financial arrangements he may have a resentment, if he is putting more money toward the living arrangements than you. This can often be an underlying cause.
Then start to narrate you experiences. Not blaming or criticizing, but rather informing. Let your mom know how uncomfortable it is to have fun, feel good, and then be given the cold shoulder by her. Let her know that it is hard for you to watch her be unable to hold on to good feelings in favor of dwelling on what’s wrong. Also let and the boyfriend know that it seems after you have good feelings with your mom they turn sour after the two of them talk. Tell him it makes things more difficult.
The point here is that the situation is not likely to change very much — but what can change are your reactions to it.
Wishing you patience and peace,
Proof Positive Blog @ PsychCentral