I have this same problem, this is how I resolved it. There are several sharpeners out there with guides that keep you working at the right angle. One is made by gatco, one is a smith(saw this one at lowes today for $39) and the other is the most popular but I cannot for the life of me remember the name, i use the gatco. Anyhow, you clamp a guide on your blade and the stone has an aluminum rod that goes through a hole in the guide and it keep s your stone at the right angle ( you can choose the angle based on application). I can sharpen my D2 benchmade knives in a few minutes with one of these, and anyone who knows anything about steel knows D2 is tougher than woodpecker lips. They will stay sharp under fairly constant use for a month, and will shave for probably 60 days before they get too dull. I mostly cut tape and cardboard so results may vary, but you get the point. I have a gerber axe, sharpened it with one of these kinda modified to fit the back of the axe head and it will do anything a pocket knife will do at this point, my shovel will too.
These things will get your tools STUPID sharp, if you lose a digit dont blame me.