Oh my, you are MAKING A BABY and right there, your body is working full-time, using energy, changing and needing extra rest. You need rest! Even God took time off after 6 days of work but I bet you are not getting any real break. This is not an option so get together with DH and figure out a way to get some major recharge time at least once a week.
Here are some things you can try:
Pay the 17 year old to babysit so you can get out of the house. Even tho she is part of the family, giving her some money tells her that you value her time and are not just taking advantage her. I'm the oldest of 6 kids and it can be a thankless position!
Have DH commit to waking up at 5:30 for your youngest so you can have an extra hour or two of sleep. DH is getting up anyway for work ,right?... so can he work your son into his morning routine?
DH can wake up in the middle of the night on weekends when he doesn't have to work. That gives you 2 nights a week without those interruptions.
Let the homeschooling go. I homeschooled 4 kids and can tell you there's plenty of time to catch up. 5 years olds just need to learn the alphabet, numbers to 100 and their colors. If you don't get to that this year, you can cover it next year.
Get together with another mom-of-many to talk and share. Sometimes it helps just to know we are not alone. Take time to develop those relationships now, even tho it's hard to make time.
In a quiet moment, share honestly with DH about how you need some relief now. It's great that he is willing to help. And it is in his best interest to make sure you don't burn out now. He and your children need to have you around and in your right mind for a long time so make the changes you need to find your stride.