Is there any prospect of you reducing your hours in your current job? Doesn't have to be a permanent arrangement. Put in a request for flexible / reduced hours and cite your child problems, which may be resolved after 6 months.
Then spend that time working on your own stress, and supporting your older child.
If you're working less, and have a reduced income would that prompt your dh to step up? If your general stress is reduced you can then consider properly whether your marriage is workable, or not. Is dh practical and supportive in other ways? From experience, it does feel lonely when you're the only adult in the house and 100% responsible for the dripping tap, the storm damaged roof, the phone call to the insurance company and the tradespeople.
Research properties that would allow your dc to still have school opportunities (I'm assuming state schools, not private), but which would give you a simpler life. Look at them realistically and ask yourself some hard questions about what YOU actually value, how you use your current space. Could you see yourself living in a 3 bed semi, with no utility room, washing drying on the radiators, no seperate study for wfh, no spare room for guests. (which is what many people aspire to!). Do your dc currently use the play room and garden, or would they be happier playing games with you in the only living room, and kicking a football around the park? Could you survive with a tiny patio, a bench and a couple of pots of spring flowers?
Generally in life, when I've felt as overwhelmed as you appear to be, I've tried to give it 3 months. 3 months of small efforts in very directed specific ways (e.g. spending time and being present with your older dc for 15 minutes every morning / evening). But also 3 months to make proper plans and considerations, so that at the end of the period you are 100% ready to take action. As so many posters have suggested, take it in bite size chunks (you and your dc first). Imploding your life feels tempting but I'm sure you can bring about a better outcome.