You are 100% not being unreasonable. What you are trying to do is not sustainable. I know, because I tried, too. It is not possible to hold down a professional job with additional money worries and limited support if you have not had a whole (or even adequate) nights sleep for multiple years. Until you accept that, and change something you will feel like this.
I know it’s hard, I’ve been there. Sleep training DS is not the answer (its more stress for you) you can’t change other people. There is no way to make a child sleep. Some children are poor sleepers, as are some adults. As of tonight, I would suggest co-sleeping. Co sleep isn’t the best sleep, but it’s better than no sleep. Crisis management. If that doesn’t help you can move on. You can change co sleeping later when it’s not the best solution and your own sleep needs are fulfilled. You can fix your marriage later if you sleeping apart is such a problem (although if your DH can’t see how you need better sleep than you’re getting, and isn’t prepared to change anything in terms of your family aims, or his contribution to help get you both through, it’s probably already broken)
Getting DH to step up may be the answer. It might mean you can continue for a bit longer without burning yourself out. It might mean you both burn out. It might mean you burn out faster, because managing another person on top of your DS is……more effort. Depends how much of patriarch DH is, to be honest. You may need to be Very Clear that you cannot continue carrying most of this, and what the consequences will be if he doesn’t change.
So look at what you can change. You MUST accept that you can’t continue as you are ‘until….’ And look at your life goals. Who’s in favour of this ‘sticking it out until we can remortage’? Is it a person who is sleeping more than you? Again, I feel your pain, but in this position you can’t optimise everything else. Your needs come first. Speak with your relative about repayment terms. Speak to a debt helpline or a financial advisor. Look at other funding options. Accept that you cannot optimise your finances at the moment, that this is a VERY resource intensive time for families, and you might just need to coast, and not work ‘towards’ anything. That you WILL need to make some compromises. Maybe that’s a different job with less childcare and putting off the re-mortgage until later. Maybe your DH takes a loan you can both pay off more slowly. Maybe you need to move to cheaper housing, get a lodger, sell a car. Something has to give here. Don’t let it be your mental health.
Also, I think I read you are a teacher? Do you need some time off sick to deal with this? You get full sick pay for a while, and it applies to mental health as well as physical.
I’ll be honest, this broke my marriage. DH was saying ‘it’s not forever’ ‘we need the big house, you wouldn’t be happy anywhere smaller’ ‘DD needs to learn to sleep’ ‘we need you to work full time’ etc etc. But, dear reader, when I left him it turned out I did not need him at all. Nor the full time job, nor the big house. I lived perfectly contentedly in a small house, which I could afford ALL BY MYSELF with much reduced hours and the ability to take a bloody nap! Now, things are different, because I am getting enough sleep, and I want different things. But I’m not making somebody else pay for them with their health, time or effort. I will decide what I have the capacity to attain. DD never did ‘learn to sleep’. (She still wakes now, and she’s 11). It also appeared that he did not need my full time salary to keep his beloved house. He managed perfectly well on his own, and even managed to keep it nice and tidy. He was not ‘too stressed’ or even ‘too busy’ to do it himself when I wasn’t around to do it. He did ‘notice’ that the dishwasher needing emptying and he even managed to (at least I presume) feed himself every night and find his own keys without a tantrum. He also kept on the cleaner (which he was completely adamant ‘we’ couldn’t afford) when it was his labour she was saving and not mine. He simply would not step up if I would do it, and he didn’t care how much it broke me to meet his HIS priorities. He said he did, of course. But, dear reader, he only SAID so. He didn’t actually DO anything. And, because it is women who get the social blame (and consequences) and who cannot bear to see their children’s needs go unmet, we do it. There is no blame attached to this statement (well, not to anyone I wasn’t married to, anyway) it’s simply how things are. My point is I was almost at breaking point, and he wasn’t, so he had the freedom to look to the future. I couldn’t deal with my present. He had dreams at the expense of my truly awful reality. It really did almost break me, and I couldn’t see how stuck I was until I was in a better place myself, and I kept saying I ‘had to’ do it ‘until XXXX’. When I was just trying to optimise everything else, and not accepting that I was being crushed and it was stupid to try and find the best solutions for money etc while I was cracking up, and something else would have to give. It’s OK if you put things on hold to get through for now.