If you have an old couch or chair you might practice tossing the ball into the chair while attempting to put right-to-left revs on the ball. If you don't have colored inserts put a piece of
tape on your ball
Track to see in which direction the ball is spinning.
As a last resort, you might consider purchasing one of the heavy duty wrist supports on the market. Many years ago my son was bowling in an after-school program. He was throwing a backup and I couldn't break him of it. I bought him a Cobra (ladies) with the metal index finger. No more backup.
Some of the things that contribute to a backup is not maintaining a straight wrist. If the wrist flexes during your backswing and your fingers rotate to 7 or 8 o'clock position at the top of your backswing and stays there, a backup ball could result. A straight wrist will either prevent the latter or will cause the fingers to rotate back to 5 or 4 o'clock at the release point. Also an armswing that is too far from your body will sometimes result in a backup ball - by keeping your armswing in tight to your body(little or no daylight between your armswing and body)will help prevent a backup ball.