Me and my partner have been together for about a year and a half, and don't have sexual intimacy super often (maybe once a week), but when we do, she always seems to finish while I can never seem to get over the edge. We haven't had sex, as we both want to wait until marriage, but have done everything else other than penetration. The only issue in our sex life is that I cannot seem to finish, and have yet to even once. I can get myself off just fine, so I'm not sure what the issue is. I feel bad because she feels like it is her fault, when honestly, I am starting to think that I am the problem. It's not that she isn't good at it, I'd say quite the opposite actually, but I can never go all the way. I start feeling like I am about to finish, and then it goes away. Is there any advice or anyone who has an idea of what is going on?
TL;DR, 21m cannot finish during intimacy with 20f partner, needs help figuring out why.