I'm trying to work a non-engineering dayjob (I need benefits) while doing a part time engineering job which involves a lot of independent decision making (we're prototyping). And give up drinking. And smoking. And fix my diet.

This is a pretty common problem with ADHD - these goals are basically "improve absolutely everything all at once and become awesome". Of course you're perpetually burning out when your objective is running a marathon today when you couldn't run a mile yesterday.

I'd say focus on getting your schedule regularized first and foremost. A part time engineering job alongside a full time non engineering job makes that practically impossible, so it's not surprising other things suffer. Any chance of making that engineering gig a standard 9-5?