Hi Sam!
I'm in a similar situation and I completely understand your frustration. I've been working out with Cathe since last October and I still struggle with many of her abdominal routines. However, i've come up with a few things here and there that have helped me...maybe they'll help you as well!
(1). I absolutely agree with Wendy about working your abs more often. Wendy gave me this same advice a few months ago and it has definitely helped! Previously, I was working my abs 2x a week. Now i'm up to 4x a week (on average - some weeks 5x, some weeks 3x).
(2). Try other instructors. I'm 100% Cathe loyal BUT I found I needed easier abs routines to help me get started and restore my confidence. I found a couple Ellen Barrett Pilates DVDs on Netflix that included abs sections. I also would string together the abs sections on Jillian Michael's 30 day shred DVD. Completing intermediate routines has helped me build up to Cathe's more advanced routines.
(3). Work your abs at the beginning of your workout instead of the end. I noticed sometimes if I was running short on time or just completely tired I would skips abs, which again hindered my progress. Plus, sometimes I struggled with Cathe's abs routines because other mucles were exhausted and giving out (hip flexors, arms, etc.). By doing abs at the beginning of my workout (after the warm-up) I made sure that I actually got my ab work done AND I found many of Cathe's routines slightly easier because my supporting mucles were fresh and ready to go. PUB is a prime example. I used to struggle sooo much with the side planks in PUB but I finally figured out it was because my arms were exhausted, not my abs!
(4). Prepare modifications in advance. Many of Cathe's abs routines were way too advanced for me so i'd end up doing maybe 1/2 of the reps for each specific abs exercise and then stopping because I was losing my form, other mucles were taking over etc. It turned into a vicious cycle - the abs exercises were to difficult so I couldn't do many of the exercises but that meant my abs weren't getting stronger because I wasn't do many of the exercises (I hope that makes sense). One Sunday afternoon I picked a few modifications for each section of my abs (lower, obliques, upper) that I knew I could do. Then, when I came across an exercise in a Cathe routine that I couldn't handle, I would sub in my planned modification. Though I still struggle with many of Cathe's abs routines, i've definitely gotten stronger. There are times when I can get through maybe 3/4 of the reps for a particular exercise, but instead of stopping I immediately start one of my planned modified moves.
(5). Try Abs Hit and/or STS Abs Circuits. I LOVE Abs Circuits! The routines are longer (average 15 minutes) but Cathe often adds in 5-8 second breaks between each exercise, which she doesn't always do in some of her other abs routines (LIC comes to mind). These small breaks make all the difference for me. The routines on Ab Circuits are still tough, but find I don't struggle with them as much because of the brief rests.
HTH!
Good luck!!!
Katie