I'm so sorry you are going through this. I am 37 years old myself, 30 weeks pregnant, and my toddler is 22 months old now. Once I stopped breastfeeding him at 12 months, we switched to whole milk 3 times a day and added solids. Now, it is only fully solids and just takes a bottle of milk before bed. Doing the above and having a routine has helped me a lot. Also, my partner helps after he gets back from work to feed and prep him for bed. Like Hollie said, is it possible for your partner to drop your baby off at day care so you can catch some sleep during the day? That could really make a lot of difference. I hope things change for you, as this can really be draining if you have to do it alone without support.
My little ones are 21 months apart I stopped breastfeeding cold turkey 18 months, got pregnant a month after she turned 1. It was becoming too much and it was like a 360 she started sleeping in her crib and not waking up it really shocked me. It is Def a lot not gonna lie and having someone there is really a big help I don't really have much help myself but I just try to stick to a routine for cleaning and taking care of them but someday it doesn't go as planned.
Oh you need to wean that toddler. If you don't do it now you'll be tandem feeding both. I mean, if you WANT to tandem feed, no worries, but I get the sense you don't want to? As long as hubby doesn't work nights, get him doing night duty for your toddler. My kiddo started sleeping through when we night weaned! You need sleep. You can't survive without sleep.
Could your partner arrange help/ drop offs to a daycare a couple of days a week to give you a break. Or arrange a nanny to come in one day a week or something to give you a break x