I'm in big trouble!!!!!!!!!?

Alright heres the deal My ex girlfriend (17) and I (18) broke up 5 months ago because she cheated on me, I wanted to forgive her but she couldn't forgive herself so she said she needed time and I was obviously hurt but agreed to this because I didn't want to lose her because I love her more than anything. But over the past 5 months we've been talking every night on the phone and we'd developed a much closer relationship. Now the problem is her best friend doesn't like me, never has and never will (for absolutely no reason at all) her friend is somewhat on the larger side of life and my friend told me to imagine next time I was out with them both to imagine her friend getting married to a chocolate cake ( I know it may not sound very funny but if you take the time to appreciate it, then its fairly hilarious!) any who I imagined this next time I was out with them which was the next day cause it was fresh in my mind and needless to say I sh@t myself laughing (not literally) and then I was talking to my ex on the phone that night and she asked me why I was laughing and I told her that my friend made up the joke about her friend and I was laughing at that I couldn't help it etc, I have the sense of humor of a 4 year old but she said I'm a pri*k and she never wants to speak to me again. and I got a big long hate message from her friend. so my question is how do I go about earning forgiveness cause she seems fairly hell bent on not giving it to mr and I can't stand to lose her I love her so much!