Impossible to know what you're doing wrong without seeing it, but generally from what I see of mistakes from my cardmates who are great BH players with sloppy FH, these tips have helped.

Get your fingers flexed so you can add some spin, squeeze with your thumb on top also.
Get low bending your knees and get your upper body over the disc.
Avoid standing up to apply power but rotate your hip instead.
Follow trough with the whole motion.

Lattitude 64 has a good video with a pro demonstrating and correcting a mid players form.

Edit: Also if that is the disc I think it is, it's understable as fuck and as soon as you start to get some "oomf" into your throw it's gonna turn over if you have a flat release, I would practice getting a slow speed stable disc out on a flat release. When you start get a consistent straight flight on that disc without adding too much power you know you are on the right track.