I'm in love with my best friend but I just hit her...please help?

I have been in love with my best friend Allison ever since I met her 4 years ago. I've tried to tell her many times but she can't get a clue. So today she came over and told me she is going out with this guy named Thomas. This guy is such a jerk and I know he will do something to break her heart. When she told me this I broke out in anger and slapped her across her face. I just ruined a great relationship and my life... please tell me how I can apologize and make it right?