Erectile Dysfunction

My boyfriend and I usually mess around every day. We don’t normally have sex and he’s not very experienced. I’m pretty inexperienced myself but not as much and it’s been well over a year since I’ve been with anyone even before we started dating 4 months ago. We’ll start to get hot and heavy and decide to have sex. We were all excited about it. I’m still iffy about being completely nude around him because I’m insecure. He wanted me to be on top today but I was nervous and took my time and it never happened. He lost his erection. This isn’t the first time it’s happened and it’s really bothering him. And I feel like a bad girlfriend because I didn’t just go for it. I wanted to so bad but was so nervous. And it’s killing him that it keeps happening. He thinks theirs something wrong with him and I’m trying to reassure him that there’s nothing wrong. I don’t know what to do anymore. I need help. Because this is starting to bother me too and I hate seeing him like this. Please. Someone help. ☹️😣😩