Your situation is completely understandable. Many people worry that they won't love the second child as much as the first, but that rarely if ever happens. So you might want to scratch that off your list, which is long enough without it.
However, all the other things you list are very real. Pregnancy at 46 is not unheard of but it's not risk free either. You are not depressed, you say, but you are clearly overworked and under-supported. You are clearly exhausted, and your are hyper-vigilant at every stage, not just for your child's development but for his well-being in this tumultuous marriage with your husband's disorder.
Your mother cannot be your only source of support due to her age - and what if she develops problems? Your husband's situation has been a matter of concern for a long time. You already have a special needs child to takes so much out of you that you cannot even get a shower more than once a week? And you are worried about more mental health as well as physical health issues in another child of an older mother.
I support you in any decision you make. It is your body, there is no "perfect family size" and don't let anyone tell you that considering your options is wrong. I have a single child, and I never refer to him as an "only" child because there are no children "missing" from our family.
I worked in women's health for many years and counseled many many women trying to decide about continuing a pregnancy. No matter what anyone says, it's hardly ever easy to terminate a pregnancy, but that doesn't mean it can't be done with a clear conscience and a whole heart. You just have to be sure - just as you really have to be sure if you carry a pregnancy to term. Unlike many women, you are looking at motherhood with open eyes - you see the benefits but you also see the strains and stresses.
I think, honestly, that you need to be able to talk with someone about this, someone who is objective and is on your side. Please call your primary care or gyn physician and ask for a referral to a counselor who accepts your insurance. See if your mother can watch your child for an hour here and there while you go, guilt-free, to talk to someone who only has your best interests in mind. Stretch it to 90 minutes so you can get a shower. It's not just about this pregnancy, but about your entire life situation. You need an advocate and you need a place to bounce some thoughts around without being criticized.
I hope no one gives you the "pro-life" argument - this is YOUR decision and not someone else's. It does not matter what some other people believe. There are far more of us who support a woman's choice and know that it's never as clear cut in someone's life as it seems from outside. Whatever you decide, it has to be informed and based on your circumstances. And I will support you 100%.