My 2 cents.
I've also gone too far.
And I've come to the conclusion that NO SINGLE ACTION, in and of itself is a sin. It is the state of your
mind and
soul when doing the action that matters. Even thinking of her in a lustful way is a sin (because it opens the door to the "spirit of lust"). Trust me on this one, that thing gets in your head and it breads.
Please, by all means rub her back. Don't rub her back and while your hands are sliding up and down her back imagine them going lower and lower.
Kiss her, hold her, spoon on a Saturday morning, and if you have the mental discipline, even shower together.

There's a challenge for a mental adept.
But what ever you do, always monitor your thoughts and know where your weakness's are.
I've recently broken a porn addiction and done tons of research on lust and the weakness's i have. You sound like a very decent kid, but because of that, know the bad thoughts for what they are when they get in your head. There are certain things that are natural for a guy to feel, but when they cross a certain barrier they start corrupting your thoughts, actions and finally your relationship. You'll know when you cross that boundary.
The people above have given very practical advice. Take it to heart.