I don't think you have lost your nerve. People do sometimes feel this way and it can be one or more of hundreds of reasons. One of the most obvious is fatigue and underarousal. Can you take a break? A real break where you can rest your mind, not a week of catching up on DIY etc, stressing yourself. I also have 4500 hours and have a new job (same unit as mightygem) I suddenly have no paperwork to do and 12 hour shifts take forever.... Sad I know, but I have started to fill my time with trivial things like crosswords, model making, reading and I feel 10 years younger. My flying is becoming fun again. I now realise just how stressed I was in the military.
If you are not stressed, you may just be bored and have too much spare capacity to think about the bad things. Get a new challenge and see how you get on.