Replying to Pains of love

hi. im 17 years old. I have been dating my gf for about 7 months now and ive definitally fallen for her. Were the typical high school couple that gets in fights, some bigger than others and mostly her getting mad at me over little shit. For the past couple weeks everything has been really great, the best it ever has been. Last saturday was one of our best nights together. Thursday night i was talking to my bro about our weekends and i kinda slipped up and told him what my gf and I had done that sat. night. So today at lunch my friend hints at my gf with some remark that he knows what went down. Tonight was the typical night (i had just gotten my new 4frnts mounted so i was especially happy) and i called her after dinner. About a half an hour into our conversation she asks if i told my friend about what happend. Knowing that lying about it was only going to make it worse, I told her that yes i did and that i was really sorry and it would never happen again. So now, she is super pissed. I feel so shitty its rediculous. I have this like twist in my stomache that wont let me eat or sleep. This always happends when we get in fights like this. Its just so shitty also because this weekend was going to be so fun but now it will be awkward and hardly fun filled. My question is...am i being a little bitch about this and letting it bother me way to much? I mean i care alot about this girl and i really do feel bad about what i told my friend. The thing that i want to say to her is, "why cant you just realize that it was a little mistake and just tell me it was an asshole move and to not do it again rather than making me feel like the biggest asshole in the world?" I can understand why she is mad but i mean WTF one person knows and they arnt going to tell anyone else. Just let it be. But i cant say these things to her cause im afraid of the worst but sometimes i think the worst might be a good thing cause eventually it would have to end.
Sorry for the rant i just had to let it out for someone to hear,
thanks