No brilliant advice, just empathy OP. Feeling the same really. All my adult life helping care for my sibling who died recently, now my parents are elderly and my teens seem to need me more than ever and it feels the career and life I wanted has disintegrated.
Relationship is strained after already giving things another go after a break, but kind of okay. Buying somewhere together is totally out of our reach in anycase, and we're overflowing in half finished projects across our own seperate places, neither of which are suitable for us to move everyone into. So a hugely sympathetic "Meh" from me.
Not sure if you're married, but it sounds like you're not (no judgement from me, I regret my unmarried position too). How is your partner with your youngest? Would he consider reducing his hours to be with him more and so you can work a bit?
2 is such a tough age without any special needs, so don't be hard on yourself, especially with the teen thing going on at the same time!
Work might be a nice break for you, as well as keeping your hand in for the future, even if it's just one day a week. Might be worth speaking to your council as you may get higher up the list for housing due to your son's needs.