LOL I laughed pretty hard when I read this

Not that you though man... this is pretty normal for alot of guys, myself included!
I remember when I lost my virginity.. I had been fooling around with my gf for a few months but she wasn't ready for sex yet. Then one day when we were messing around, in the heat of the moment she told me "I want you inside of me... NOW!". Needless to say, I was so shocked, and so excited, that I slid in there and I maybe lasted like 2 minutes before I just exploded LOL. I was pretty embarrassed secretly, but it ended up being OK because she didn't know any better, and she was in a bit of pain adjusting to intercourse aswell (as she had only had my fingers in there before that).
Just keep having sex regularly and with time and practice, you will learn to last a very long time. At times I have even had trouble being ABLE to come, because I had trained my body to be able to withstand the intense pleasure for quite a long time. This is completely normal, and nothing to worry about man

Unless it happens every single time you have sex, then you might have a slight problem with premature ejaculation. But since you are new to this, I wouldn't waste any time worrying about it! The thing that helps the most in this situation too is having a partner who you trust and who trusts you, who will make you feel comfortable and not laugh at you or put you down in any any while you are learning. Your girlfriend sounds pretty understanding, so just go with the flow and keep at it until your a pro. Congrats on getting laid btw LOL