Hey there I'm new on here so be nice :)

I've got a problem that's been bothering me for some while, on previous encounters I thought it was the time and the place...But I've been going out with my girlfriend for around 3 months now, and we've decided to take it a step further sexually. She has had sex before but I myself havn't. Out of all my group of friends I am the last one to loose my virginity, and seeing as I am 17 I feel like I'm quite old to still be one. I have wanted to have sex for some time now, but I didn't want to just do it with any one..as a drunken mistake, a rebound etc etc. So when it comes down to it, we are both having abit of a play, undressing each other etc. But the most I can ever get when it comes down to it is about half an erection; maybe even 3/4. So it's quite hard but not hard enough to penetrate. We've triend twice now, and I've tried it with another girl once before. But it's really starting to bother me now, as it's getting quite embarrising/ annoying. Wondering if you could either tell me what's wrong with me, what's wrong with what I'm doing and/or tell me how to fix it?

Thanks alot