This is tough to diagnose without video of your technique or a really detailed description of exactly what body movements you are making in order to try to steer your board.
There are a couple of things you said that may give hints as to what "could" be going on. First, you went to a stiffer boot. I have found that stiffer boots inhibit my ability to dynamically flex my ankles in order to use inoculation to use board tilt properly and I have to revert back to inclination and lean way over the edge to tilt the board. This can create that "slipping out" phenomenon you describe.
Secondly, you switched bindings. Double check to see if your boot is centered on your board. It is possible that you may have the binding mounted too far back toward the heel edge of the board. Additionally, check the heel cup adjustment of the binding to make sure it does not allow the boot to sit too far back. You can have the base plate of the binding centered just fine, but if the boot sits back too far, it creates the same issue.
If your equipment checks out fine, then it must be a riding technique issue and the best way to figure it out is to get video or be very detailed in exactly what you are doing to try to initiate your turns.
Thanks for the advice! I've learned a lot from reading your other posts that give out great advice.
I spent the day today going back to basics and it made a huge difference. I also changed the angle of my back foot from -9 to -12 and felt much more comfortable. My back foot was no longer pushing itself forward and causing the back of the board to swing forward. Got a few more days left so def going to keep practicing the turns.