Can't help you here. You're in a situation where you have deep feelings for your partner. To me, sex has always been (even as seldom as it's been) casual. Hell, if I ever end up in a relationship, I'll probably be asking you for advice. Don't know if that is going to happen, since I'm currently sticking with the philosophy of never sleep with the same woman twice. Fucked up as it may seem, the only sex advice I can offer anyone is to share some laughs and conversation. Just try to relax. I do recommend some alcohol, but not to excess.
Throw all the preconceptions out the window. From what I remember and a few guys I've talked to, what you hear is often like stereotypes--maybe some generalized truth but it doesn't pertain to every guy. For example, if you can remember the movie "American Pie" with the scene with Jim and Nadia in his room, they were making light of the fact that many guys are extremely quick on the trigger the first time out. But this is not all guys. I have talked to more than one fellow that kinda had a hard time cumming his first time and I can relate somewhat, as it was by no means instant. Hey, there's a very good chance you won't last long...but you never know. If you nervous and have your mind set on satisfying her and are worrying about shit, you might last 30 minutes. You might not be too bad either. There is no rule that says your first time has to TOTALLY SUCK for her. You'd probably be unlikely to wow her with orgasm after orgasm, but she may well get some pleasure out of the deal. Most likely, you'll need some time to adjust and adapt to each other. Some guys are probably more gifted than others, but any two people should be able to improve their sex working with each other. The guys that brag about having shagged 50+ women and made every last one of them cum repeatedly are pathetic. They are either so full of themselves and insecure that they have to brag such bullshit to make themselves feel good or they're so full of themselves that they're clueless to fakers.
Have fun.