I'm sorry you've had a couple of bad experiences with your teachers, hopefully it will be third time lucky.
There are often things that my teacher wants me to do in my lesson that I can't get the hang of straight away. If I know what needs doing but I just can't do it, I sometimes say (after trying several times) "I can't do this yet, but I'll practise it at home and I'll be able to do it eventually". The "Yet" makes it more positive than the show-stopping "I can't do it", end of. On the other hand, if the reason I can't do it is because I haven't grasped how it should be done, then I ask my teacher to explain or show me again. Saying I'll be able to do it after I've practised it at home also enables us to move on to some other productive learning in the lesson, and usually the problem is something that I do learn how to overcome, either by the following week or over a period of time. Perhaps you could try using the "Yet" word with your next teacher to forestall any further problem on that front