I believe PGA tour players only make about 50% of their putts from 8 feet so don't be
too
hard on yourself.
Before I play I like to practice a lot of 3-10 foot putts (in addition to lag putts) as these are the key distances where my skill can help me sink the putt. I like to start in close around 3 feet just to make sure my putting stroke is on. Then I will work back from there, trying different angles and different holes. On putts in the 6-10 foot range I try to go through my complete putting routine that I'll use on the course.
I also prefer to only use one ball to practice putting, preferably the exact type of ball I'll be playing with. Using just one ball is better for me because I focus more on each putt. It means I can't just rake another ball over if I hit a bad one, and also that I can't rely on muscle memory for the speed.