I would prefer you to apologize profusely for committing to something when you forgot you were already booked. Tell her how very much you really want to go to this game.
Say that you will make it up to her with an evening out someplace nice. Don't forget and do it soon. Also, if she is going to the party anyway, I would give her a small gift the day of (a nice orange gerbera daisy--they are available in Van now). I mean, its a small gesture and you did mess up.
Also, since its a party its likely to go late. Is there a way you can go to the game and skip the afterparty to be with your GF? Good compromise.
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh