I'm not sure if this belongs here, but I need help. [warning: possible triggers]

I recently started seeing this girl and it's been amazing. I haven't been in a relationship before so everything is new and exciting. Yesterday, things are finally getting really hot and heavy and she goes down on me. This is the first time I've had a blowjob, except from a prostitute (read my post history if you are even slightly curious about that comment...). The problem is, I couldn't cum. I wanted to, it felt amazing. I just couldn't hit the apex and actually climax. And I feel guilty about it. I went down on her first and made her cum; so I feel like by not cumming, I made her feel like she wasn't doing a good enough job.

Is there some reason I'm not cumming? I don't think it's nerves, though that's what I've been told by friends. I also haven't fapped in maybe a week which is usually enough to get me really sensitive. Help? I want to cum....