It's natural and healthy to react emotionally to life's ups and downs. You wouldn't want to be a robot. We can't choose the times when these things happen to us. Of course, we have an obligation not to overreact in front of our friends and family and ruin an occasion for them, but if they are truly our friends and family, they will understand that we will be sad when bad things happen to us, angry when unfair things happen to us, and happy when good things happen to us. The only time to worry is if we can't let go of these things after a reasonable amount of time has passed, but it helps a person to let go of things later if you allow yourself a reasonable expression of your emotions at the time things happen.
I know how you feel, though. Logically, what was done to your car was already done. Why make it worse by not being in a good mood during what could have been a happy time? Same kind of thing happened to me during a tournament last weekend. I received what could be some bad news just before the tournament, which caused me to shoot bad and act sad, I guess, because one of my friends commented later on that I didn't seem to be myself. I would have rather have just put it out of my mind and joked with my friends during the shoot like I usually do, but if I start kicking myself now over that, I'm just perpetuating a bad mood over something that is now one step removed from the actual thing that caused the problem in the first place.