How do I move during sex, with him or like bang together?

okay well, I had sex for the first time recently. it was in the back seat of his car and I was really nervous. we done a 69 then decided to have sex. he was kindof having trouble getting hard, and I felt really bad. he has had a lot of experience so that shouldnt be normall right? he put his penis inside of me and it didnt rally hurt except for when he went really hard and deep. but the problem is that it kept on slipping out. it had to have happened more than 5 times and it was extremely frustrating for the both of us and I felt really bad for some reason. was it my fault? was it because he wasnt hard enough? am I the reason for him not being able to get hard? once he got one erection and he cummed, he coulnt again, and that was also after I got naked so was it my body that turned him off? I am not as fat as some people but am definetely not skinny, and I have rank boobs, well I think. so was it the probability of a non-hard penis that caused it to slip out, or the fact that I wasnt moving properly? I didnt no what I was doing, are you supposed to move with his body or against it and like bump up against him?