wow get a load of these guys nutting instantly
well i suppose its better than not nutting at all
the nutting instantly thing has happened to me before, not on my first time either. I was just seeing a new girl who I thought was insanely hot, and played hard to get for a super long time (she would jack me off three times a day and never let me kiss her, or sit on my dick for a tease, looking into my eyes and never put it in; this type of shit for months). so as soon as I eventually got to do it with her, surprisingly enough the first couple of times were fine then I guess I lost confidence or got overly excited, and maybe the next five or six times in a row I'd make it past foreplay, then nut in a matter of seconds; I even remember one traumatizing time where I nutted literally instantly - like one thrust in - I didn't even know my own dick could do that. I remember feeling devastated; I was younger then and in my head I thought I was some sort of champion thus far; I had heard stories of people nutting instantly but thought they lived in some kind of alternative dimension I could never relate to. then now that was happening to me, holy shit. that was scary but maybe five or six attempts later I overcame the problem by just developing confidence over the girl and everything was fine from then on.
about not nutting that's happened to me before too but that had more to do with me figuring out my own limits. having sex three or four times a day sounds amazing when you're 17 but not so much when you're 30 and in a long-term relationship with the same girl for eight years (that's not to say I wouldn't do it, but it's physically harder and the excitement isn't as frenetic). the act itself can feel so amazing you don't always 'have to' nut.
whether position you're in, it's all in your head, really, probably depending on the vibes you get from the girl that might or might not make you worry too much. just handle the moment and with experience you'll be fine.