People usually love or hate the first yoga or tai chi class they take. If they hate it, they usually blame the teacher. But very often, the reason some people have a bad experience with their first mind-body class is that they don't understand what mind-body learning entails.
The teacher cannot quiz you to see how well his or her teachings are working. Unlike, say, a math class, your job in a mind-body class is not to do your best, it is to do what gets YOU results. You don't have to do every technique exactly the same way the teacher first shows you how to. You also don't have to do techniques for as long, or in the same order, as the teacher shows. In fact, you don't have to do any technique you don't want to do.
But it is very, very important to understand why you SHOULD change your class experience, and why you SHOULDN'T.
If an exercise hurts, consider why. Is it putting stress on an injury? Should you be going slower or for less time? If so, you need to figure out what will work given your circumstances.
If an exercise is difficult because you have a lot of tissue tension, or a lack of mobility from non-use, then the exercise is probably helping you. So long as it isn't making your condition worse, you should stick to it and put in the work. Don't push yourself too hard, but challenge yourself so you can improve.
In the intimate class setting of a hospital, it is easy to get tailored treatment and personal feedback. In a large public class, the teacher can't think about each student, they have to focus on meeting all the demand for their instruction.
To get the mind-body class experience you need, make sure you pay attention to both parts of the mind-body yin-yang. The body half is about feeling how the exercise is affecting you, and adjusting so that you feel progress without pain. The mind half is about accepting the challenge that the teacher is giving you, and using your will power to keep doing the exercises even though there is a part of you (and everyone around you) that would rather lie down and just watch TV.