why is sex weird now?

Why do I get uncomfortable during sex with my husband? I mean it feels good, it's more like a I don't wanna talk dirty or be assertive kind of feeling. I DID that when we first started dating and somewhere along the line I stopped and then it became "weird" to me. I feel embarrassed. We have been ttc for 15 months now, all testing is normal...sex is just not fun anymore and I don't have that "close" feeling with him anymore. I love him. He's a trooper with all of this and doesn't let it affect him TOO much but I know it bothers him. Is my marriage doomed? Am I alone in this?