I did something stupid and now my friend hates me

I was a part of something stupid a few days ago and I admit it was an insensitive and rude thing to do. Me and my friends got carried away and not of us meant it maliciously. Most of my friends accept this apology as they know it's out of character; in 7 years I have NEVER gotten into an argument - I know that's difficult to believe but I am not a confrontational person. If I'm upset by something I keep it to myself, and I tend to stay out of conflict. However, one of my friends has said she will never forgive me for what I've done, and says the trust between us is broken. She has every right to be upset, it was a silly thing to do. I know to most people this will just sound like an argument that will probably get resolved in a few days, but for her to say what she did is a big deal, especially in our relationship when we have always gotten along like a house on fire. It's my last year of school as well and I don't want things to end how they look like they will end, especially after 7 years of a great dynamic between all of us. I just don't know what to do. I've apologised for three days straight now, she's read my messages but hasn't replied. Do I need to give her space or should I keep pestering her? I just feel awful, and it's not something I'm use to. I've always been on the receiving end of stupid things happening, not the other other way around and I hate myself if I'm perfectly honest. I've tried speaking to her sister who's a year below us, but she says she doesn't want to get involved :L I just don't know what to do