The hardest thing is that people don't understand how tough it can be. It is relentless and it can be hard to convey to others sometimes how hard it is. People think they understand because their partner works long hours, or went away for a week for work etc. It's not the same.
The financial pressure, career limitations, curbs to social life, etc. are all yours alone. You can't be sad or ill, you just have to ignore that and get on with things.
When the kids get sick, you're with them, but then fall behind with work (and housework)
Everything takes twice as long because there is only one person to do it all.
It would be much easier if one person could do homework while the other one cooked, or one person could clean up while the other one got the kids to bed, or one person could take the kids to them clubs while the other person does the food shop, but all those things that coupled-up people can do in tandem, you have to do in series, so the day is just very, very long. Every day. People will tell you to give yourself a break, but it is impossible to have a break from it as things still need to get done and there isn't anyone else who is coming to do it.