I feel your pain. I was in your spot, working full time while parenting a 3 and 4 year old at home. It's fucking brutal. Covid has caused this. I see countless parents now who are at breaking point. You can tell just by the look in their eyes that everyone in the family is going stir crazy.
All I can say is that, for me, I had to quit my job to cope. It wasn't that I couldn't cope with being a Dad, it's that I could cope with trying to be a Dad AND working a full-time job. The other thing is that, myself and my wife were not spending ANY time with each other. We were just mum and dad, and forgot to be husband and wife.
I appreciate leaving your job might not be an option, so all I can say is that the end is in sight. With society opening back up, nurseries and schools opening, things will get better.
Remember, you're not weak and nor should you feel guilty. You're trying to cope through a worldwide crisis. Anything short of mass hysteria should be considered a success!
Good luck to you and your family.