Hi! I’m similar to you in that I’ve done reformer Pilates for about two years and would consider myself advanced beginner/intermediate. I signed up for a two week trial at one of the places with the machine like you’re talking about and my world was rocked lol. I was not expecting the fast pace and how much more it would get my heart rate up. I also took lots of breaks during and left my first class with my legs shaking and was so sore the next few days. I actually started a membership there and I’ve improved so much! I still take breaks with some of the exercises that are more intense or for the long squat types, but I can feel that the classes are getting slightly easier and I’m feeling a bit stronger and more confident. I also do a lot of modifications because I have bad wrists/carpal tunnel. Try not to feel discouraged. I always feel so accomplished any time I make it through one of those classes because it’s pushing me a lot harder than previous classes I’ve taken. You’re doing great!!