OP, I 'get' how you are feeling. My Ex wanted to change from his 8-5 job to one that involved split shifts, never being here to help over dinner time and bedtime, leaving me to try to cope with it all, despite the fact that I'm disabled, and so are two of our 4 DC's.
I couldn't do it. I just couldn't. I felt that he cared more about his 'happiness' than the practical aspects of family life. I felt that he was 'checking out' of the family, leaving me to do all the 'grunt work' involved in family life.
I had no family help, no help from Social Services, HomeStart only help you if you have a DC under 5yo.
All I can say is that in the end, I felt that if he cared more about his work than his family, and I had to do all the grunt work anyway, then why add resentment on top. I resented him in the end for checking Pitt of OUR family. Yes, in an average family, shifts like this are bearable - but in a family where one parent is disabled AND one or more of the children are disabled, it's another kettle of fish entirely.
Loving someone who is disabled and having a child with a disability requires a lot of sacrifices.
The OP is struggling to cope physically with working. To have no help when she gets home is likely to result in, long term, her health deteriorating further and being unable to work.
This was NOT his only job offer. He has prioritised the job that would give him work satisfaction, despite the fact that there was a job offered that would have brought in more money (thus relieving the pressure off the OP who is disabled to keep earning at all costs), and had hours that fit in better with HIS family life.
I don't think the OP is selfish - I think her partner is. It doesn't matter that the amount of hours worked are the same - it matters that the time spent helping his disabled partner with the things she needs help with has reduced.
Help is only help if it is given with what the person NEEDING the help needs help with IYSWIM.
The OP needs help with the post 5pm things that have to be done in her family. If he is helping by doing other things, but NOT what the OP ACTUALLY needs help with, then in fact he isn't helping the OP at all.
He is doing stuff to ease his conscience at NOT giving the OP the help she actually needs, rather than taking the job that means that he IS present in the house to provide the help the OP needs.