Hey Tarantula,
Don't worry about your periodic "poor" performance....try to be thankful that you are able to ride ....an activity that you love.
I've been competitive at tennis for the past 15 years and it used to be a true passion until I tore both menisci in my knees....now I can never play tennis again...I can't even twist.
The good news is that I've turned my attention to the gym and cycling which include some of the best activities for my body and their enjoyable.
My point is just pace yourself, don't be so hard on yourself and definately ENJOY THE JOURNEY and you'll be much happier.
Later.
Tenacious1