OP, it sounds like you feel stuck and hopeless as far as your options. I have also been an unpaid carer for 30 years. I've done little bits of paid work in between on and off, but I've given up the career I could have had. I didn't want to stick my child with a paid carer when I could do it myself. I know some people don't have the choice but we did, and that's the choice we made.
What have you done while you've been home caring? Have you been involved in any community groups or organisations? You might be surprised how you can leverage this experience to get a paid job. Maybe sit down with an employment advisor and they can help you identify all these skills and experiences. Jobs with a carers' service will look on your lived experience very positively.
I know there is still the issue of care for your 18 year old. Your ex-husband should be responsible for 50% of this. If he won't, maybe look into part-time day programs. It might be good for your son and he can make some friends there, if that's in his capacity. Now that your circumstances have changed, you may have to look into having someone else support you to care. Paid carer? Family member? I know it's probably not what you want to do but sometimes, you just have to to make ends meet.
You have so much valuable experience. Could you do a short course? Where I am, there's always work to be had in aged or disabled care fields. I've got degrees while at home while full time caring. There are a lot of distance supported options these days.
Just some thoughts OP. As I posted before, social services and employment agencies will be able to help you find your options and how you can make them work. I know it can seem overwhelming and daunting, so having someone to hold your hand during this process is so helpful.
I also agree with other posters about getting legal advice on fighting for half the house.