(((((((((((((((Vetswife)))))))))))) I'm sorry it's so hard right now. I understand to a degree. My husband died active duty and it's hard dealing with everything by self. I can only imagine adding an adult husband child to the mix. We want the best for our children, I do understand that.
Sometimes though, if we don't take that time for us, we aren't going to be able to be there for them. I understand not wanting to leave the children with someone for a couple hours but maybe there are some things you can do at home. Do your younger children go to bed at the same time or take naps? Perhaps you can set up somewhere in your house, even just a corner in a room with a comfy chair, some nice candles, books, whatever you find soothing. Maybe you can talk to your older children about certain times when mom just needs some down time.
Your 11 year olds might not totally understand but maybe be firm and maybe even have a calendar so they know what day you are going to take some time. You could put on the calendar, one day a week in the evening (or whenever you think would be best- even if not completely ideal). You can even put the time on the calendar and explain to them that mom really needs this time so she can be a better mom. Let them know that you are available to them at any other time, but on this calendar day, between these times (It could be an hour or whatever you can do. Even 15 min of quiet time helps), they need to stay in their rooms and play quietly or lie on their bed and read a book, etc. It might be a struggle at first, but they will get so they understand and will comply.
This will help not only you, but it will show your children some boundaries at the same time and all done in a loving way. They will learn how to take time outs themselves so they can better cope too.
The one thing I have finally learned is that I can say all I want that my children need to take time to do self care, but if they don't see me doing it, they aren't going to either.
I don't know if this will work or not, but maybe worth a try? Hope I didn't step on any toes. I just understand that overwhelming feeling with no solutions in sight. Please keep us posted and good luck. You're a good mom and it's obvious that you love and want the best for your children.