Being up at all after 13 sessions is good for your first month. It takes much better than average play to break even against the rake.
Stopping after losing 20BB is fine. You should never lose more than what puts you in a bad emotional state for playing. Aside from leaving once you lose your limit, you should leave when you are tired, tilting, generally don't feel you are playing well, or can't find a good table to play. If your emotional state is good, and you are at a sweet table, you should stay no matter how much you have won. If you are good enough to beat the game, variance is more likely to put more chips in front of you than to take it away. The chips and cards don't know you've been winning. If you try online, a .10/.20 game or a .25/.50 game are similar in many ways to a live 3/6 game. I think at Paradise there is no rake for .10/.20. The .10/.20 in my opinion is about the same looseness as 3/6, but worse after the flop. The .25/.50 is a little tighter than the live game preflop, but about the same skill/a little worse than live postflop.