OP I'm going to try to be really kind to you and encourage you to be kind to yourself as I'm a single parent too and I think that some (including some staff at schools) don't see the coal face of the reality of that one in the middle of a global pandemic and I've been home schooling since March.
Mine is 15. Start of term is always a hurdle, don't know if you are on a low income, I am and no matter how much I try to budget over the holidays and for new school uniform there is always something unexpected comes up.
Plus you are the main emotional support for your kids. This morning I was so emotionally drained and tired I went back to bed for an hour and then had a bath. With the door open. In a small flat with a teenager and I don't want to moan, but I can't remember the last time I had an unhurried wash or bath. Honestly it feels like the first time since March.
I've put on weight too, and really neglected myself because I've been so busy holding things together.
So I'm trying to tell myself that start of term is a load off as at least now I've got some other people i.e the school staff to manage half the day with my daughter, provide challenges, and it feels like forever since I had some time to myself.
So I do get it. It's no wonder you are feeling drained.
Single parent reality as you know is once they get back to routine things get a bit easier. This year for them and us is way more emotionally demanding than it usually is. But in a couple of hours we will have done the first day. They will be tired at the end of it and maybe get to bed at a decent hour.
You are doing really well, do try and find some time to yourself.
I need to think through improving my health here. Seperate thread...