First off mate, I'd say that if you're worrying about being shit in the sack, it's probably making you be shit. In other words, it's a self-fulfilling prophecy.

Now it's all very well and good me saying "you need to improve confidence in your own abilities" but this isn't real advice, let's be honest. I think the best I can do is to give you a few pointers that you'll notice make a big difference (hopefully!).

Firstly it's worth mentioning foreplay here. If you're doing foreplay then there's a good chance your girl will enjoy herself. If not, start doing it - you've got to preheat the oven before you stick in the turkey! (Quote Michelle, American Pie). Foreplay can be a ridiculously easy way to get a girl turned on, and a good source of info is (believe it or not) lesbian porn. Almost everything I learned about arousing a woman before I actually ever had sex was learned from lesbian porn. So get watching and get learning, and then get experimenting - all girls are different and it'll take a some experimentation to get it right with your girl.

Secondly, be dominant. Girls love to be dominated by a guy because it brings out his raw masculinity. It takes an alpha male to be dominant and controlling in bed, and this is a huge turn on for women. Try doing things like initiating surprise sex in the most innocuous circumstances. Pin her arms above her head whilst you do stuff to her. Girls go fucking wild for it.

Thirdly and finally, mix it up and find techniques and positions that work for you both. You'll know when she's really enjoying herself and when she does, you'll start to as well. And if you enjoy sex, you'll become more confident and better at it.

Hope this helps dude.