I can see why you envy the split families that you describe...I envy them too.
The trouble is that very few lone parents are in the position of your 3 friends.
Many more are like me...ex has had the DC today, from 10-3, for the first time in 2 months. He has taken them to McDonalds, fed them Smarties and let DD2 have a sleep on the way home so now she'll be up til midnight. While they were out I worked my way through the washing and ironing, changed all the beds, mowed the grass, walked the dog, cleaned bathroom and kitchen and took the garden rubbish to the tip.
He has just dropped the kids off, and gone back to his girlfriend's. Now I need to feed DC, bath them and try to wrestle them into bed. They are over excited because super dad has bothered to do something with them for the first time in 8 weeks.
You may do this all week, but you have company, help and support over the weekend. I'm not saying it's not hard for you...I'm saying that it's hard for everyone (except the twunts that walk out on their kids and choose not to see them).
Perhaps if you offered to babysit for some friends, they would return the favour. I often have my DSis's kids for the night and they she will have mine if I want to go out? Because sitting in your house thinking how much easier everyone else has got it isn't helping YOU to feel happier.