Hi Nals,
This is a major change in your life, and one that it sounds like you were not prepared to make when you first got married.
It is perfectly understandable that you feel overwhelmed.
Add in the fact that you did not have the birth experience/ hormone changes to physically connect you with these children, and there you go.
You expected to be a grandmother. Play with them, spoil them some, rev them up and send them home.

Instead, the rules got switched midstride on you.
You say you cannot relax at home while the kiddos are at school because your hubby works from home. What about joining a club or special interest group doing something that you enjoy? Or going to a gym to get some exercise (real exercise that will help your body get rid of all the builtup stress chemicals in your muscles - chasing after toddlers doesn't count).
Also, if your husband works from home, then he is in the ideal position to
help you with housework, chasing munchkins, etc. If he says there is not the money to hire help - then he needs to step up and help out, too.
Even having just 1 day for yourself a week will be tremendously helpful in restoring yourself. Point this out to your husband. Asking for 1 day out of 7 is not too much.
I had a nervous breakdown and was contemplating suicide when my youngest child was 2 yrs old. I actually spent a few days in the hospital. We learned the hard way that getting overwhelmed
can be serious.
One of the things my counselor suggested to me was putting my son into daycare part time so that I could have some of that alone time. I was also dx with clinical Depression and started taking medication for that as well. But the simple time to myself made a HUGE difference.
As far as the children go - this stage will pass. In just a couple of years they will be off to Kindergarten. Two years sounds like a long time when you are under stress, but it is a light at the end of the tunnel. They will eventually potty-train, they will sleep through the night, they will grow up.
In the meantime - feel free to come here and vent as needed and share with other parents, and maybe find an interest outside of kids.
Good Luck!