I have two disabled children - one who I hope will be able to attend university if they choose and have a career eventually, and who is a young carer for their sibling who is more severely disabled - so I know how hard it is to get help. I also have major gynae problems and other chronic health issues. So I absolutely get it.
But your 2 year old is your responsibility, not your 18 year old’s. You chose to have the youngest, in spite of the situation with your severely disabled child (assuming it’s not a recently-acquired disability) and unfortunately with being quite a bit older than you were when you had your 18 year old, you’re more likely to face your own health challenges than before.
You would have to manage if you simply didn’t have the older daughter, or she was too unwell herself or left home. You would have to fight for help, but right now it seems it’s easier to let your 18 year old babysit for much of the time so you don’t have to. I know all too well how hard that fight is but you’ve given up because it’s easier to keep your daughter at home to parent your toddler for you than it is to take on that battle.
I would never want to limit one of my children’s futures because they have been made to feel responsible for the other. It’s not fair at all. Your reproductive choices are not your daughter’s problem.
If you are struggling to access support then make a post and people will help. You should be able to get carers to support with your disabled child which would free you up to look after your 2 year old more. It’s not necessarily easy but it is possible. But social care will look at how much help and support you have so if the current situation is “it’s ok because I use my 18 year old as a nanny” they will see you as needing less help from them than if she weren’t around.
If you don’t free her of this responsibility and let her go, she will likely resent you and potentially her siblings too, when she realises that she’s missed out and limited her potential because of a situation that was not at all of her own making and never should have been put on her. You will destroy your relationship with her as well as her life. You need to put her needs over yours, here.