Hi OP. I think if you think your job has been contributing to your mental health struggles for 2 years, then it's time for a change, isn't it?
You've just been off on long term sick - what is going to improve when/ if you go back? - There's a risk things will just go downhill again if your job is contributing so much.
You shouldn't stay in a job that makes you unwell, unless you absolutely have no other option.
You have to make a change at some point if there is going to be a chance things will get better. So I say go for it and do it for yourself - don't worry about letting people down.
One thing I have learned about jobs since being in my 30's is that everyone is replacable. If you leave a company they will just replace you - seriously. It's harsh, but they generally don't care, and when you realise that, it's quite freeing, because you can do what you need to do, for you.
Put yourself first and go for it, what have you got to lose?
:)
(From someone who has been off sick for a period and just about to start a new job!)