I need some urgent advice

My boyfriend and I live together and we study full time, he wants to have sex all the time and I am just not feeling it and i dont know what to do because he seems to get frustrated and upset when I say no. 
I love him so much and he isn't the problem I just don't want to have sex at the moment because I'm tired and stressed from school and family issues. 
I really don't want this to draw a wall between us and cause fights. 
Help please...