With a problem like you've described, again, I try to focus on the essence of the problem, and construct an exercise dealing specifically, and only, with re-establishing a "correct" habit. As in the other answer: Play as slowly as you need to to play perfectly, and watch your thumb carefully while you go TITITITITI. Don't let yourself do it wrong. Make sure every note sounds good. As you get used to that, "raise the bar" by adding speed, or doing it as part of a roll. Do not start playing a piece or doing licks. That would distract too much while you're trying to establish a new habit. Over a number of practice sessions, stay steadfast, literally watching your right hand as you do very easy moves, gradually increasing the speed or difficulty of the moves. If you revert to your bad habit, "lower the bar" by doing something easier/slower so that you "always play correctly". Again, play "as slow as you need to to play correctly". In time, you'll be able to add more and more different moves/speed/distractions, and retain your good new habit.
As with the other problem you asked about, changing a long-term habit requires persistence and patience. It is not the most fun part of practicing, and certainly not the reason you took up banjo. As you slave away at creating the good new habits, go ahead and give yourself a break once in a while and just have some fun with the instrument, then after a while, get back to work. The method I've suggested definitely works, but it does take some discipline over a period of time. So hang in there!
Good luck, Sam. Let me know how it goes.