Has anyone been in a similar situation? How do you set boundaries without feeling like the world’s worst human?
I've done it. You have to be very determined to take care of yourself first and all your basic needs before you think about helping others. Otherwise resentment sets in, as you've found. You could set boundaries similar to how I did.
I had a needy friend taking up all my time to the extent that I wasn't getting enough sleep even. So I decided I was going to focus on my evening routine and chilling out before bed, from 8.30pm, which meant no taking phone calls after 8pm and getting people off the phone by 8.30pm (I went with honesty for that part, "have to go now, I need to get ready for tomorrow and go to bed"). I figured if it was a dire emergency and someone needed me desperately, they'd either send a text or leave a voicemail explaining a little of why. That's what I'd do if I really needed someone's help and it couldn't wait. Plus I could make exceptions if it was someone I felt like talking to or someone I had plans with that night. Everyone else could get a text/call back the next day at my convenience.
I also set a time limit for this person of half hour, after that I'd invent a reason to get off the phone. Left to their own devices they'd be on the phone hours and also be calling me up for such pointless stuff as wanting someone to talk to while they locked up their house (to leave it temporarily for running errands or whatever), so someone else could reassure them they really had locked up properly. I wasn't willing to listen to that sort of thing, so I'd say I was busy right now and I'd call back later, which I did.
It had started to feel like they were treating me as a full time on-call member of staff, just unpaid! So I set boundaries that stopped letting them monopolize all my time and emotional energy. I wanted time for my own stuff and for giving to other friends.
Unfortunately they didn't take well to the new boundaries, taking offense when I ended calls before they wanted to. They obviously decided they weren't getting what they wanted from our friendship any more so they faded themselves out of my life and eventually just stopped calling me. Based on that, I'd say don't worry if you want to ghost her because she'll probably end up ghosting you anyway!
But if you want to try to stay in touch, just decide on your availability both in general and specifically for her, then set boundaries accordingly. You don't have to have it out with her, just start being less available. Also change the subject when you are talking with her, so it becomes a pleasant chat about something or other and not always you listening to her problems. If she won't respect you and takes offense or keeps changing the subject back to her issues, then you don't really have a friendship anyway and she's just using you. Then it's not a situation of trying to fix a friendship, it's a situation of removing a user from your life.
You're not being mean, she can get online delivery if she can't go out. Most things concerning daily life can be dealt with online these days if necessary. It's different if she had to take a child to A&E and asked you to watch the rest. I'd say she's using you as an excuse to get away from them. I imagine she's used to having a nanny as well as a husband, if she was well off.
I'd put less effort into looking after the children when you do choose to help her out. So long as you're not neglectful of their basic safety, who cares if her house is trashed or they get upset arguing while she's out? You're not her nanny or being employed to entertain them. It's her job to raise them so they don't misbehave whilst she's not there. Do what she does instead, go on your phone and leave the children to get on with everything like they usually do, putting on the TV for keeping them occupied or something simple like that. If they need to eat, give them toast. If they need to go to bed don't bother with bath time, teeth brushing or bedtime arguments. These things don't matter for the odd day here and there. Just simplify everything. Don't be afraid to ask her for the odd favour of some kind in return too.