Here's a comment to similar question that I posted elsewhere... maybe this will help you.
Remember the first time you ever strapped in? You probably sucked riding regular, too. I did. So did pretty much everyone else. You got good at riding regular only through days or weeks of practice and repetition, developing your muscle memory, board feel, balance, etc.
If you have no control it's probably because you're favoring your back leg (which is normally your front leg -- there's that pesky muscle memory doing you a disservice!). You want your weight forward when riding regular and likewise for switch. But you're very accustomed to weighting a particular leg, and this habit is impeding your ability to ride switch. When you're in the back seat, you lose the ability to effectively control your board. (Think of all the beginners who, when riding regular, instinctually "lean back" like they're afraid of the hill, and weight the back leg which results in crashes).
When teaching yourself how to ride switch, you're gonna have to break it down to baby steps.
First, start riding regular and actually pause to reflect on what you're doing, and why (if you can't mentally break down your motions, you're going to have a hard time trying to reconstruct that technique to switch stance). If you can't do this, do not proceed to steps 2 et seq. Get a lesson and tell them you are specifically interested in learning how to ride switch.
Then, apply it. Take those steps (where do I weight, how should I properly initiate a turn, then how do I link another turn, balance, etc.) and very deliberately put them in to practice.
Third, once you're OK linking the pieces, force yourself to ride an entire trail in your switch stance. Do it until you can ride the entire trail without falling or stopping. You can go as slow as you need, just don't stop, stay in motion.
Fourth Once your comfy riding switch, pop some 180s at low, then moderate, then higher speeds and make at least a few turns before reverting to forward stance. Get used to riding switch at higher speeds. Then go back to step three and ride an entire trail a little faster.
Lather, rinse, repeat. Just like riding regular, practice makes perfect.
Do these slowly at first. Preferably on blue terrain. You will fall a few times. You will probably be able to negotiate a trail within 1-2 days. With repeated practice, you'll increase confidence. Shortly, you'll be decent enough that you don't fall anymore. Then it's just a matter of practice, practice, practice as you increase the speeds at which you're comfortable.