Would it ever be worth going back to an old job that ruined your mental health?

Slowdown

Only you know. 

I was in teaching for 28 years. The last two were in a terribly toxic and bullying environment that saw several extremely competent teachers leave the profession altogether. I know all about crying in car parks. 

My counsellor believed that experience along with other life events have led to complex PTSD and I would absolutely never consider going back. It was 6 years ago. I had to take out a personal grievance against a member of senior management and it was traumatic. 

I feel sick when I think about it. And angry too. 

You are obviously competent and employable so treat yourself better and look for an environment that offers support and balance and a professional working ethos. 

Ozzig

I have colleagues who left as they really did not like the way they were treated, then a year or so later returned.

When they came back they knew exactly what they did and didn't want from the job and set their stall out from the off, it has worked really well for them.

Their new roles are not that different from their old ones, however now with appropriate pay, and expectations/responsibilities (on both sides) set, they are much happier.
The things that were taken for granted and drove them away are now recognized, rewarded, and kept in check so they don't end up back to square one.

Depending on how much has changed, you are best placed to know how they would welcome you back and how they'd treat you the second time around.