I'm 10 years sober. That sick and tired feeling is your body trying to repair itself. It will go faster if you quit entirely. There's a lot of good writing on the process, there's a good "pink fog" bump a few weeks in, and then its hard for a few months (ymmv, everybody's different etc. etc.).

As another poster said, it's likely that you haven't done any long term damage YET. But do consider stopping entirely.

Don't look for an easier job. At 40 it's somewhat normal to be outperformed by younger people until your wisdom/experience kicks in, but that may not happen unless you stop completely.

Good luck. You've taken the hardest step of acknowledging you have a problem.