Exchanging I love you's

I've been with my boyfriend for over two and a half years now. We still haven't said the three big words. Although I know I love him, I fear that he may not feel the same way. 
I'm not putting a time limit on this, but everytime I try to have a conversation about it, I clam up. I honestly don't know what to do, and sometimes I get the feeling that he gets tired of me after spending two to three nights with him... Am I just being paranoid??