I think this topic has been covered many times. I've always wondered if the balls are worn out after so much use or if they're DESIGNED to be shorter. Either way it doesn't help you judge distances on the course.

I bought a little black note book. I made a grid with each row being a different club. Each time I take a shot (and I can take a reasonable estimate of the distance) I write down how far I hit the ball. I might put down "t" for a tee'd shot or "r" for out or the rough etc. It's helped a great deal.

As you get better your distances will change too so this helps kee track of that aspect too.