Dear Pea,
First off, thank you for posting. It takes guts to own up to what you did. You aren't a rapist and since you were both 15 you certainly aren't a pedophile. Secondly, you were a kid yourself. Our brains keep changing and maturing up up until our mid-20s. On a very real and physical level you are *not* the same person as you were then. This is not an excuse or a story. It is scientific fact.
It sounds like you crossed a line/ make your partner uncomfortable. Some people would say you committed sexual assault. It was not monstrous, but it was wrong and regrettable. It was not so terrible that you deserve to feel badly about it forever. Try to speak to yourself like you would another 15-year-old boy in that situation. You might talk sternly to them, but you certainly wouldn't want them to be hurt or locked away. You'd want them to learn.
As such, I honestly think the best thing you can do to make moral amends (and restore your own self-worth) is to help teach others, especially kids, about good consent practices. Here in Britain there is a charity called Sexpression that does this - maybe there is something similar where you are?
I agree with you that now is not the time to talk with your friend about what happened since she is undergoing a period of grief. If you do decide to write her an apology or ask if she wants to say or do anything with regards to what you did, then please do so after a few months. Also, I would strongly encourage you to read this booklet first:
http://www.phillyspissed.net/node/36I seriously respect you for trying to address and be accountable for this. The fact is that there are hundreds of thousands of ignorant kids who have done the same thing you did (I did something similar when I was several years older than you were) who tried to justify it to themselves. Some people may even say they want to execute/ torture rapists, while having done something similar in their own past [so don't automatically trust those who shout the loudest online]. Over the last decade there has been a gradual shift to an understanding that sexual assault and rape do not always (or indeed, most often) involve physical force or a victim who is passed out.
One of the best pieces of writing I've read online about this is Doone Woodtac, who says:
To top all of that off there’s the vast gray zone between being labeled a rapist and not being a rapist. The stigma of the word makes it really hard for young men to own up to non-consensual sex. Rape sounds like such a violent word, and when men reflect on events and note the stark absence of violence, they may admit it was non-consensual, but they will not call it a rape. Even though those things are equivalent. Men who don’t take their victims by violence or overt coercion struggle to identify the event as an act of rape. For them, rapist means something else, something that’s clearly wrong, something that’s obvious. For them, rape can’t simply be having sex with someone who isn’t struggling against you, sex with someone who just lays there without participating, sex with someone who hasn’t shown interest, but who appears to allow it to happen. I think most of the time, these are what typical rapes look like.
The vast majority of people who have committed such an act will repress it or find excuses as to why it wasn't that bad or illegal. The fact that you have chosen not to do that shows you have integrity and moral strength. As such, you cannot be a monster. There is goodness in you. Hold onto that and try to spread this message to others when you can and you will feel this goodness and peace expand and keep you floating above the dark waters of anxiety and guilt!

Very best regards,
Sprock