I agree with frzl if you are playing your "A" game and getting suckedout on which happens at the lower limit fequently 20BB is a little low. Also when running good and the tables are good you should play as long as possible again playiing your "A" game. If your winning but feel like your starting to make bad choices, or losing focus take a break from playing go watch a video for your limit or re-read arrticles.
When playing your best it doesn't mean that your going to win all the time. Realize that you need bad players in the game and they will suckout on you, maintain your goal, play your best, study and study some more.
Setting limits $ wise in my oppinion is not the right appoach, which is to results oriented. Better to set limits on time, break your sessions down and play for a certain amount of time. When you have reached your time limit click the autoblind key to off and sit out your next blind. Then go over your sessions in pokertracker or elephant or holdem manager what ever progam you use, find hands that you think you had a difficult time with and post your hands for the coaches to have a look.
Good skill to you, see you at the tables