I can't lose my virginity

My boyfriend and I tried to have sex for the first time (first for me, not for him) and when he was inserting himself, he said he couldn't get it in fully because there was some sort of resistance.

It was also terribly painful on my end and I already expected that to happen but for some reason, my body involuntarily "closes up" and "blocks" him even though I don't want it to.

I'm 100 percent sure that I wanted to be intimate with him and I kept assuring him that I want him.

I just don't know why my body is resisting him even if mentally, emotionally I was prepared for it? Is there something wrong with me? Why can't I lose my virginity? Is there anything I can do? Please help.