How to turn anger to happiness quickly?

I've gotten really frustrated with my girlfriend rescently. She's confusuing me and threatened to leave me but then says she doesn't want to (which is adding to the confusion). I haven't talked to her face to face in over a week and in the time in the middle just casual texts. I'm not really and angry person at all, If I am angry I normally go very quiet. This time however I completly lashed out and destroyed a park bench (nobody saw me do this but I'm walking with a slight limp...it was damaged before anyway). I'm really angry at my gfs friends as they have told her loads of S *hit about me which she is now starting to believe. I'm meeting my girlfriend on Friday, I want to be happy and relaxed so I don't say/do something stupid, and so we can re-build our relationship from this setback. Anything like relaxation exercises or something to calm my nerves?