[vorpalhex]

You are in burn out. It's not that you aren't intelligent or capable of the work, but it sounds like you just haven't clocked out in a long time.

You can fight burn out for a short period but you can't delay it forever. You resist burn out by working out hard, engaging in stimulating non-job work (eg a hobby) and engaging in intentional relaxation.

Ultimately you need to find a way to take time off.