There is some really good advice here and I can't really add to that. My only comment would be to ensure that you remain positive, do not give up and do not give the slightest hint to your employer that you are thinking of quitting. There are times when a positive attitude will get you a long way. It once saved my job when all of those around me had given up.
Stress has a large part in performance reduction.
My main stressor is thinking that I don't know something well enough. Detailed preparation sorts this out. If you go in to a flight or exam with confidence your performance is likely to improve. When I was doing IR, I broke down the whole flight in to sections and made sure that I knew the keypoints to pass that section. Anything in addition was a bonus. IMPORTANT.IF you get something wrong don't beat yourself up about it at the time, forget it and move on. You cant change it. Recognise the problem...talk about it later and improve by not making the same mistake again. Keep it simple...the airline backed you with a job...back yourself and you can keep it! DO NOT GIVE UP!
Best of luck from an older bloke who hasn't got as far as you have!