It’s obvious that you should ring fence your deposit, as everyone is saying.
But my view is that there are much deeper issues here and it’s not quite as simple as the fact that he is taking advantage of you (although he is and has been). OP, I actually feel quite sorry for you - you’re only 32 and it sounds like you’ve had very little fun or freedom in your young life.
You were barely into proper adulthood (yes, I know you’re technically an adult at 18, but questionable if you can count being in full time education as fully-fledged adulthood) when you became involved with a man several years older with two young children.
You say you’ve never wanted your own children (and that of course is your prerogative) but from a very young age you took on the responsibilities of part time parenting and the attendant sacrifices for someone else’s children. That makes no sense to me if you weren’t interested in children. I also think that the constraints of the situation mean you’ve never even had a chance to explore the issues fully and possibly change your mind about having your own children or wanting to get married.
You state that when you met you lived in a flat share and earned minimum wage as if that somehow equalises your and his situations now. It doesn’t really. You were doing then what is normal for a recent graduate getting their career trajectory underway. At that stage it’s all about potential as you’ve since proven with your now decent career and income. He, by contrast, was several years older with heavy commitments that were not going to reduce for many years. I can’t imagine what attracted you to him or the situation, but obviously there were reasons.
Unfortunately I think the PP who mentioned the boiled frog analogy is spot on - you’ve spent almost your entire young adult life doing more than your fair share to support this man, his children and (indirectly) his ex-wife - emotionally, practically and financially. It’s gone on for so long you think it’s normal and now can’t see any other way your life could or should be. I have daughters in their early twenties and frankly would be very sad if one of them got into a similar situation and that was their whole future (even though it would of course be their choice).
My honest advice to you would be to extricate yourself completely from this situation before you get any older. I’m almost certain you won’t do that because you say you love him and his children, they are your family, and you’ve all been through a lot with his illness. No doubt you feel obligated to see it through because of loyalty but also because you are stuck due to the guilt that would ensue - you know that while you could walk away tomorrow and be fine, he would struggle hugely to pay his rent, provide for his children and continue to pay his ex-wife.
One final comment based on my own experience - taking on such a load for so many people might feel fine at 32 when you’re full of bounce and energy but becomes less easy at 42 and positively difficult at 52 when you’re menopausal and exhausted. I’d bet a large sum that in 20 years time you will be very fed up with the endless burden.
Do you plan to support his children through university and/or other life stages? He doesn’t earn enough without your input and his earning potential has prematurely stalled due to his health. How old is the ex-wife? If the same age as your partner, do you plan for him to be supporting her for decades, well beyond the children growing to adulthood, because she can’t work due to her own health issues?
I’m not saying you should leave him, and I’m pretty sure you won’t, but that being the case, these are things that you will have to factor in to your planning.