Girl, puh-leeze do not be so hard on yourself!!! Push/Pull and Legs & Glutes are as tough as you make them. If you have the DVDs, try doing some of the premixes, which can make them more of a challenge. For Push/Pull, try going as heavy as you can. I used to think this workout was a breeze, but going heavy really makes this one effective. And because Cathe uses slower reps (usally a 2/2 count) and because she does only 1 set of each exercise, you can really go heavy. Give it a try! With Legs & Glutes - I like to really really focus on form with that one. Because she goes lighter, you can really go low on the squats and deep on the leg presses to get the most out of the movement.
Now, for the cardio:
As I've said in previous posts, my first Cathe purchase was the CTX series. I did The Firm, Gin Miller, Keli Roberts, Karen Voight and could breeze through those. I ordered Cathe because Collage deemed her 'advanced.' Great googly moogly - they weren't kidding! I couldn't finish Step & Intervals, Power Circuit, 10-10-10 - any of the CTX workouts! And I thought the weight lifting segments (though Cathe said they were "lighter/high rep workouts" were extremely heavy and tough! I did the CTX series religiously for several months. I lost 4 dress sizes and ended up being able to complete each workout with a few modifications. My next purchase was the Rhythmic Step, MIC, Imax DVD. I couldn't make it through the Imax warm-up (those power scissors!) - but I kept at them - and they became more doable and more doable overtime. Just keep at it! You're doing such wonderful things for yourself! You're improving your self-esteem, your bone density, your cardio capacity, your coordination - so many things.
Never, ever, ever give-up. You'll feel SO INCREDIBLE that first time you do one of her step workouts the whole way through without stopping or modifying!!
(And I think the Classics would be a great way to build up the cardio endurance needed for her later, tougher workouts.)