I agree with ashley. If you think you will mess things up by having sex with someone...most likely you will and you are not ready. Sex is slightly more then just swapping fluids and fucking. It is a deeply emotional psychological, and yes spiritual experience. If you are not 100% sure that you want to do something sexual, then don't.
Normally there is a point in your life were you just have to take a leap of faith, trying a new food, going on a roller coaster, or even joining a sport. But with something as big and as...deep as sex, you need to just know. Not know as in "Sex is the next step for us, we've been dating for X amount of time, been on X amount of dates. I know it is time to have sex." You need to Just KNOW it. Best case scenario is that i works out find and you love him and he loves you. Worst case scenario is that you leave the room emotional hurt and scarred from this and it echos out into your life.
And on a side note off the pulpit, I have been on the receiving end of what you are going though. My Fiancee sometimes have to push me off her because she is not in the mood or something is bothering her. Some days she doesn't even want to hold hands. It's tough for me, I will admit, but eventually I just sit back and let her have her space. But it is better just to let him know before hand and early that you don't want to do anything or that you don't want to do anything past this point. If he is a decent man, he will understand
All the best, William