Hey dude
you've been with your girlfriend for a long time i'm sure you trust her and she trusts you, so you've no need to be embarrassed. Give it time, it's just anxiety. Take things at your pace and let it be natural. Spend some time relaxing and being together before you actually try to have sex.
I guess you're young, so you're not allowed to stay over at hers or vice versa, right? Well that helps a lot... If you guys slept naked for a while just so you get more comfortable with each other. Talk things through. You'd be surprised what an amazing weight off your shoulders it is to just talk about things. When you're embarrassed, tell her. Tell her how and why you're concerned and you'll soon find you've overcome a hurdle.
I wish you the best of luck..!