What I am getting from your post here is that you are thinking about his needs an awful lot (exactly what you want from a balanced relationship) but you don't talk about him reciprocating in any way.

Let's cut to the chase here. The problem is not you by the sounds of it - it appears to be your husband's behaviour.
Mental illness or not, his behaviour appears to be at the root of your unhappiness.
The question is whether he will seek help for this or not.
You going to therapy alone isn't going to help, in my opinion.
Does he know the destructive effect his behaviour is having on you?
If so, has he agreed to get help?
Your marriage seems out of balance and that needs urgently recitified before it becomes serious.
As a husband he really owes you this.

All marriages go through imbalance from time to time when one person starts to lean heavily on their spouse. I've been there myself many times over the years. When that leaning starts to actively destroy their spouse however someone needs to say "Enough" and the other needs to listen and react positively and constructively to re-establish the balance. In that sense you are probably not going through something millions of others haven't been through as well. You do need to get on top of this though by talking to him. Be tough if you need to. The alternative doesn't sound particularly appealing.

I know I am in the minority when I say this but I would not recommend that you take on a PhD with those grades.
In all likelihood you will find someone willing to take you on but consider this. Over two degrees, you have not shown any technical excellence and in fact have demonstrated in your exams that you have consistently only learned about half of what you were taught over presumably a 4 or 5 year period. This is not an encouraging place to start a PhD from. The process is hard enough without having to relearn half your degree. Leaving aside the technical aspects, the essential soft skills of resilience, determination, drive, motivation etc are usually demonstrated during the degree as well. Can you show evidence of those? High grades are very strong indicators of all of them so you will need to look to your extra curriculum activities for that.

If you have neither of these I would urge you not to go down the PhD route. There is a very high chance you either won't make it or you will run out of funding.

Of course, i am not telling you what to do. You are welcome to disregard my advice but if I were in your position personally i would not be considering a PhD.

That's a good point ToL about learning how to ask the right questions.
I also don't think it is the supervisor's responsibility to ensure the student has no omissions in their work or to even tell the student about them. I always saw it as my responsibility to find out what was expected from talking to other students etc. and researching online. I know some supervisors are very hands on but making that an expectation is risking the student abdicating some personal responsibility and ending up in this situation. The student is therefore wasting time and energy trying to figure out how they can apportion blame to their supervisor when really they need to be focussing on what they need to do to recover the situation. IMO that latter action is what separates a good researcher from the rest of the flock.

There is no point complaining or sounding bitter. That is just a waste of time and energy. If your skills are not attracting the job you want then you need to retrain. It's as simple as that and it doesn't matter what age you are or what qualifications you have.
I am not sure what you mean by a real job. Cutting grass is very much a real job in my book.
What is it you actually want to do, what skills do you have for that job and what have you tried so far to obtain that job?

Virtually everyone suffers imposter syndrome at some point.
I forced myself to stop caring what other people thought of me because I realised I couldnt control that.
You might want to stop white knighting your supervisor as well. These people are not as intelligent as you think. No human is. You are psyching yourself out.

Agree with the above. I am not sure why anyone would want or need to know this sort of thing.
I would certainly never talk about my wife to that extent with anyone.

Congratulations on making this decision and getting a job offer.
Your last sentence tells you that you made the right decision.
This was never about "failing" or "giving up".
It was simply about recognising that you were doing sonething you didn't enjoy.
It's great to hear that you feel so much better now.
Good luck for the future.

This is a risk when anyone takes on a new role where they have some power.
I certainly would never have wanted a new academic in my viva but it doesn't mean they are all like this.
New academics should have to go through training with an experienced person in the viva with them until they can be trusted to do the job properly in my opinion.