I think it's a common thing when you're first in the saddle. It's a new experience, everything feels really good etc.
I have 2 bits of advice - which most likely echo's many things already mentioned:
1 - Stop panicking... The more you worry, the more likely it's going to happen... at which point you resent your own orgasm. If you're not enjoying, she certainly isn't going to...
2 - Do everything you can possibly think of during foreplay, make her come every way you can think of... at least then, as someone else said... you can do as you like, she's happy.
3 - If she is putting pressure on you to hold on, that's only going to make it worse. Again, it'll just leave you panicking, scared of doing what nature designed you to do
Tell her that even though she may not mean to, that you're feeling pressure - and that pressure can only make things worse. Show her this thread if you must.
just relax, take loads of time over foreplay (mainly focussed on her) and then enjoy... and don't pump too fast