In my own personal experience, you have ups and downs. Your instructors will know of the training curve, where at some point in training, people take a step backwards, or don't progress as well as they have done in the past. It is a normal part of training.
However, this should not happen all the time. I would have a word with your CFI about it, and explain how you feel. Constant negativity is not productive, and the CFI should be able to work through the issues you have, which may mean changing instructors, changing the pattern of instruction, having a break, etc.
The more you progress, the tougher it can get, and self belief and confidence are essential to making it through - and these should be instilled within you by the training regime.