I bought the Bow Trainer 2 and a half months ago, and it works, but it's no miracle. The full draw bands only go up to 70# at my draw, so that's part of it.
When I was just starting out with hunting weight bows with training wheels 15 years or so ago (low letoff), I would draw on target, hold for 10-15 seconds, and release. Try doing that in sets of whatever you can handle and work your way up.
Mind your form and muscles, if they start feeling sore, especially in the wrong places, let up for a while. Damaged muscles are highly counter-productive.