I cannot forget the event that took place
Why can't you forget that event?
I reacted too harshly to my girlfriend's family
Now that you realize that you overreacted, you should apologize to your girlfriend's family. Be more patient in the future or you could be making enemies out of your future in laws.
It's like you are a mind reader. That is exactly what I'm planning to do in the future.
Yes, it's best to soothe hurt feelings right away. That way you can move on and hopefully forget it.