Rick,
Like everyone else has said - moderation. But I don't agree with your saying to let the car and racing set for a long while. It's a passion and something you obviously love to do, and to stop doing it doesn't make sense. You just have to learn to balance the your life and hobbies. Relationships take sacrifices of some kind, but sacrificing what you love most doesn't make sense to me.
Instead of giving it up totally, try dedicating small periods of time instead. Does your girlfriend sleep in late on weekends? Then get up early and tinker with the car for a hour.
Compromise. Keep your girlfriend AND yourself happy.
