There is a saying that stress = responsibility without control. To me this looks exactly like your situation.
just wanted to say that this is something i've not heard phrased like this, and it really clicked for me. in my case, i'm stressed out about other issues but the situation is the same in that i have responsibility but no control.
last weekend i read the stoicism book that MMM recommends, and one of the techniques the book talks about is explicitly dividing the items on your plate into three piles: those you can compeletely control, those you can partially control, and those you can't control at all. for the ones you can partially control, the author suggests that you internalize your goals. like, when playing a tennis match, focus only on playing the best you can, which is entirely in your control in a way that winning the match isn't.
for the last year, i have been feeling physical and emotional stress like never before, mostly due to the fact that i am responsible for my grandma but her health and her actions are not under my control. it's a super poignant and difficult situation for me. and, like the OP's situation, it's not going to get better. this has been really hard for me because like you, OP, i am a problem solver. i get things done. but there's nothing for me to do here deal with both the logistical headaches and and brutal sadness of watching a once-vibrant person flicker and fade out.
since i read the stoicism book, though, i've been focusing on MY part in it, which is really the only thing i can control. whether she forgets her pills, or whether she tells her helpers she doesn't need any groceries when her fridge is bare, or whether she falls in the middle of the night, i have no control. but i can control how i react with her and how i manage all the things involved in her care. surely i can control my own mind, at least.
so, yeah, this technique helps me. might it help you, too? sort out the piles of control and focus only on the stuff that is actually yours to manage?
another thing that helps me is to sit down and consciously mine my stressful experience for whatever good stuff i can find -- like it's focusing me in a clear and tangible way on preparing for my own old age (financially, physically, emotionally). it's giving me lots of practice in giving feedback to people in a pleasant, non-confrontational but still effective way. it's teaching me about how to communicate and have a relationship with someone who cannot hold up their end of the conversation (something that is difficult for me). stuff like that. helps me see the stressful experience for what it is, which is a gym for making me a better person/bigger badass.
are you learning anything here? could you be if you switched your focus up a bit?
finally, the technique of negative visualization can really help re-set your expectations so that you focus on the half-full part of the glass. what if you had no job? or what if your boss was a sociopath as opposed to merely weak? what if all 10 of your employees were princesses? it could always be worse, and realizing that makes it easier to appreciate the good stuff.
i guess my major suggestion is to read the stoicism book and try it out. :) it's helping me a lot so far.