Why do I start to ride good when it's time to go h

That's pretty normal if you don't ride on tracks ALL the time, like two or three times a week. Your first sentence explains it all. When your riding trails, you are out to have a good time and relax and enjoy your self right? When you go to a track, it is automatic to think " I have to get around this track as fast as possible" because for the most part tracks are meant to be "raced" in most cicumstances. It happens in the exact order you said which is your not 100% comfortable at first so you fight the the bike, which makes you extra tense, which when your tense causes arm pump because your not relaxed, and then you add being tense with arm pump which causes you to fight the bike even more which tires you out 5 times faster than any thing. Then towards the end of the day, when your more comfortable with riding the track, you don't ride as tense and your arms are already tired from riding all day so they don't pump and you ride much more relaxed for alot longer time. It's kinda an aged old problem, and the only cure is to ride all the time, it's simply a matter of being comfortable on your bike, which takes alot of practice. Hope this helps.