The most likely reason you're doing better while high is due to the fact that, like many drugs, pot will inhibit certain reactors. Pot mainly causes either a relaxing feeling, or a paranoid one. While you're relaxed, the fears of screwing up no longer weigh so heavily, and you're free to let your muscles go to work. Once that sense of "oh well" starts wearing off, every mistake will register.
Shorthand, if you hadn't smoked pot before playing once, you'd probably be playing that well normally, but that isn't the case anymore.
If you're going to keep smoking pot, but want your sober game to improve to the same level, I'd suggest music. Everytime you smoke, have something happen, like the same band playing. It can work with other stimuli, but music is simplest to control. Then, after you've done this a good amount, try playing with just the "high" music, not the high. It should put you in the zone, or at least closest to it.