I had one of those days last week on the day of my CPL skills test. Went up on a flight beforehand to practice a few things, (limited panel, timed turns, unusual att's etc) and just couldn't get them right. Landed feeling totally deflated with the situation, so decided to scrub the test for another day as by then I just wasn't in the right frame of mind to take an important test.
A few days later, went and did the same flight again beforehand, got everything more or less spot on, landed feeling much better and much more confident, took the test and passed.
As a pilot you will make mistakes along the way but you cannot dwell on them, you have to accept it, and move on - as the a/c will still be moving whilst leaving you further and further behind which can then lead to more and more mistakes as you try to catch up with it. You also have to make tough decisions too - for me to cancel my test was a very tough decision, but speaking with the CAA examiner, he agreed that it was the best course of action for me and it was a good decision given the circumstances.
If you are making multiple mistakes, rather than trying to correct them all at the same time which might then lead you to carry on getting things wrong and feeling more and more deflated, try to get one thing at a time right on each circuit, then on the next, try and get something else right, and gradually build yourself up, then when you do get that first thing right, you will then boost your confidence, rather than it taking another hit and you will have learned from the lesson rather than feeling it was a failure.
Ask the instructor to do the radio for example to begin with, to let you concentrate on flying the a/c, this will reduce your workload and you can still learn from listening to what he is saying and where in the circuit, then gradually as you build up, take over the radio when you feel you have enough spare capacity to handle it.
