Like you, I started out with the firm and "thought" I was in decent shape. I "graduated" to Cathe about 2 years ago. And... like you... I had to modify her workouts - a lot! Don't get discouraged! It takes time to work up to the level that most of the folks on this board are at (I'm still not there). Take the time to notice the small improvements you make - week after week, month after month.
And make sure you don't overtrain. Your muscles need time to recover. Wait until the DOMS in your legs goes away before attempting lower body work again - the healing process is what's building the muscle for you. I wouldn't do GS Legs more than 2x per week (at most).
Last comment - when I switched from the Firm to Cathe, I did start using protein drinks. It may have been mental, but I do think they helped me build some muscle early on (it won't help the DOMS, though!)