Afraid of orgasming

So my boyfriend (m20) and I (f18) have been sexually active for a while now (he's the one I lost it to) and every time we do anything together, when it gets to the point where it's too intense of a feeling, like I could probably cum if he kept going, I have to stop. I don't want him to because it feels good obviously, but like I get this rising intense fear inside me and I have to ask him to stop. It's frustrating because I want to be able to finish, and it throws off the mood and I feel like it makes him feel bad. This isn't a trust issue or anything, I've been with him a year and a half and I love him very very much and everything is consentual and happy.

Now for context, I have only ever tried going in on my own once and I didn't like it. Usually when I masturbate I just rub on top and focus on the clit, but even then I can't get there. (Sorry if that's tmi) it freaks me out in a way I suppose. For some reason when I do things with my boyfriend, even if it's the exact same thing, it doesn't scare me and it makes me feel good and loved and safe up until the breaking point.

I don't know what to do about it. The last time I was almost there, I made him stop and started shaking and crying. He comforted me and told me it was okay but I think it is starting to affect him. I don't want my own sexual fears to drive us apart.

I haven't experienced any sexual trauma, there shouldn't be anything holding me back psychologically but there is. What can I do?