How do Freelancers deal with poor mental health conditions?

Work yourself to the condition you are able to drop them. Your mental health is far more important.

I did it in September with a specific client, giving ample notice (more than double the notice stipulated in the contract) and I haven't felt better in months. Not just because of the initial relief but because I also realized that I was justifying things about my behavior in response to the environment that weren't healthy to begin with. I've also been asked to stay on for an indefinite amount of time as a possible problem solver for a day a week in case they needed me but I replied with a flat out no. I needed to sever that cord.

Also consider speaking to a professional if you can afford it, especially if you're dealing with multiple issues you can't seem to be able to juggle by yourself.

Therapy is so underrated by people like us that are supposed to be pros in knowledge working that... well don't let me get started :D. I can't find it now but I read a tweet yesterday that said something like: "pro athletes train their bodies everyday with coaches to deliver high intensity performances, when's the last time that a professional in tech trained his mind with a professional?"

Hope this helps. Take care Matt!

I've been a freelancer for a couple of years, and although most of my customers have been really nice I've had two customers that are very exhausting.

Unless there is a contract that prevents you. I'd say the best way to fix problems with clients that affect your health is to search, and find another client to work for. Take the chance to get out if you find the opportunity. If you are professional about it and give your current client notice in advance, you'll leave without damaging your reputation.

I just jumped into a new 4 month contract to get out of a tiresome long term contract.