I have a few concerns about your odds of re-hooking up with this girl slug. Don't take this too critically, as there is really no bad way to lose your virginity, unless it is in prison.
First off, the car is a pretty bad place to do it. You mentioned the heat and everything, that is going to leave her with a fairly negative memory of it. Also, there are two type of girls, the ones that want to reach the O and aren't concerned about your O, and the ones that expect you to reach your O. You missed on both of those. It isn't a big deal that you didn't get her to reach, that is tough to do, but there might be some concerns by her that you didn't reach.
Did she know this was your first? It would probably be good for her to know, the first should be treated as a mulligan situation. If she knew and she doesn't give you one more shot, then she isn't worth your time.