I had to break up with my now xp2. I have 3 DC ages from 2 years to nearly 7 years! I have ME (CFS) as well. There have been times at the end of last year (seem weird putting last year) My Mum helped me. I was really ill and had to go in to hospital. I sure you will have someone for back up if you ex does not help you. If you really need them.
I am doing ok being a single mum. I thought it was going to be really hard and I would not cope. But I can cope. Things will get easier for you as time goes by.
The hardest bit in my life atm is my xp1. He is making our lifes hell. Most likey b/c he had to see his DSs. (long story) He did not want to be seem as not wanting to see them. Hes doing it all again now, saying I wont let him, when it really he does not want to see them.
I know its a hard thing to do, but let your ex do what he wants with the children. I dont mean mess them around!! but let him see them but if he does not want too then dont try and make him.
I have found out the hard way what ex`s can be like if made to see their children. Its easier if you just dont bother to try and make them. My xp1 now tries to turn everything on to me. It much easier to look after your DC yourself, rather than making your ex help you. That just makes life hell and even harder to bring up your DC.
Dont let him push you or Dc around, with one minute seeing them and then the next minute not. Days and times need to be sorted. There will be times when they need changing. for you and for him, but try and stick to the time table. Remember that he has the DC on x days/times and he must do the same back for you. If you can both do this them single life for you can be fun.
Try not to fight, its not good for your DC and its not good for you either. It only makes life hard. My mum (step mum) told me a lot of things that have helped me. She said the more you worry about getting your own back, the harder it gets. I have I learnt that the hard way and I was not the one that made xp1 see his DSs
Wishing you all the best.