Speaking as someone who has had guys from both ends of the spectrum, too soon is way better than longsexrape.
I think this is something that a lot of younger guys do not get. There seems to be a general consensus that long sex=good sex. As you're saying, it's not really the case.
Some of my most mutually spectacular sessions of intercourse have lasted 5 or less minutes. It was/is all about the buildup to it, and how intense, pleasurable the act itself was. For whatever the duration was.
I've been with girls that wanted marathon sex, but it usually involved 10-15 or so minutes or intercourse, followed by oral, followed by a position switch, more intercourse, etc, etc. The "marathon" aspect had little to nothing to do with drawn out penetration.
My current girl, I can get her off with a combination of penetration and pressure in 2-3 minutes. Some girls it can be like 15-20 minutes. But, an hour or something like that? Not so much, at least in my experience.
I have also had the aforementioned "longsexrape" experiences. Especially while on suboxone. Pounding away for 45 minutes, trying to get off, while she either dutifully helps me to an end or in some cases I could see that it was becoming or was painful. Drawn out sex can be a nightmare.
My point is..... the duration of intercourse doesn't have a ton to do with good sex. Obviously, you want it to be more than a couple minutes. But, you strike me as someone that is just going through a phase where you're psyching yourself out, and you'll be fine in short order.
The key is if you're both getting off. Be happy you're able to. As a sometimes opiate addict/former suboxone prescribed human, there's been days where I yearned for the ease of teenage sex.
If your GF's chill about it to begin with, I'd be pretty sure she's willing to brainstorm on how you can help get her off before or after you do. If she's not already getting off from the sex.
If you can get her off with oral, your fingers, whatever, it will take pressure off you. It won't feel like everything has to be centered around the sex. Less pressure combined with some of the techniques other posters have suggested will get you to where you want to be.
You'll be fine. I'm pretty sure of it.