How do i get over a bad experience?

Hi! A couple months ago i was in a relationship, he was my first boyfriend and also the first person i slept with. Now i can't think about sex without crying or feeling nervous/panicked and i don't even exactly understand why. I feel immature and weak for feeling like this. I am not currently in a relationship nor am i actively looking for one but i want to get over this before anything because i don't want to be a bad experience for my future partner and ruin things. Are there any resources that could fix me?