It can be worrying when you feel like this and want to know why. Sometimes it is a phase that you are going through and no particular reason for it. Or you are maybe over tired or need a holiday to completely get your mojo back.
Everyone feels like it sometime in their working life and you could try and look for a new course to get stuck into. It may not be work related , it could be something pleasurable like Tai chi or art classes.
Unless you want a career change you will need to get your attention to detail back where it should be. Would you be able to get some emotional help and talk things through with a trusted family member, or a very special client that you really know well. Ask them if they have noticed a change in you becasue often other people notice our unhappiness before we do.
Sleep is very important as it can have a bad effect when you don't get enough, are you waking early and staying awake?
I am glad you have put your thoughts into words it is good for you, and I will hope to get your reply soon
RosieR