Ok Fin's here.. Everything's gonna be ok now! I feel really bad for you dude, since you sound just like me. I'm not sure how old you are but when I was at school, nearly up till the end, I was very, VERY insecure about myself, not at all confident, and especially around girls! It sometimes felt like torture. I think many guys are like this. Many aren't of course and seem to naturally just spawn girlfriends like a manga monster spawns tentacles when he finds a virgin! It's sickening to watch but us shy guys must suffer it all the same. So here's my simple tips to hopefully help get you out of this situation...
1) The time for
THINKING is now over. Ever had the feeling you've thought about something over and over and over? It's easy to do. STOP going over in your head what could happen, how nervous you should be, what she might say, etc.. etc.. Just walk up to her and ask her if she fancies going out with you as your girlfriend. Say something like "I reckon we'd have fun together", and give her a nice smile. And kablam! Before you know it you'll be sat in the back of cinema with her wrapped around your hot dog!
2) You can worry for half your life about stuff that never even happens!

And then worry about how much time you wasted worrying!

Give yourself permission to say NO to negative thoughts. Just ignore them.. move on, think POSITIVE only and more positive things will come your way. The occasional worry or negative thought is natural, but then just acknowledge them and move on..
3) You are a very cool guy. Stop thinking about this as
CONFESSING. That is the wrong word to be using for this. You have nothing to confess. In fact, WHEN you ask her out, which I think will be very soon, you will be doing her a favour, right? Because you're gonna be good for her. You will not be confessing, but
OFFERING yourself,
the man of her dreams! Think positive!! Place yourself on a new stage! Command your future!!
4) So what's the worst that can happen? She says no. It would be a shame but not the end of the world. How best to think about this? Well, if she says no then it's obviously not "meant to be", and frankly anyone who turns you down is not ready for you, or doesn't deserve you. Make it work for you.. don't let it knock you down. But it sounds like (4) won't apply, since she sounds pretty interested to me! So I say, go for it, and sooner rather than later or you may miss your chance. You only live once