Has this ever happened to you?

I'm 15 and have a Brazilian friend who gets in fights regularly and has trained in Hapkido for a number of years back in Brazil. I've been taking Taekwondo for 4 years and just started kickboxing and have been taking Qi- Kong for the past few months. Today we were just screwing around in cl as we clinched, then started hitting each other although I turned around to make sure the teacher wasn't about to come in. I took many hits and after people were taking about how he could beat my ***. I felt a sense of dissapointment because not only did I not fight to the best of my ability, but I felt I didn't show what I could really do. I was sort of half *** fighting as Forest Griffin puts it. How do I not do that? Have you ever lost a fight that you could have won? Hoe do I remember what I am truely capable of when I'm in a confrontation?