Charlotte,I dont know much about your situation but that could be a good thing-lots of mums feel like you are,even those with partners 2 tiny children is very hardwork add to your mix your exp and no wonder you are feeling down.
In a way you are grieving the loss of a relationship,the loss of a life you hoped to build with him.
In truth (easy for me to say) the bloke is an ass to treat you like that!!! but we girls dont learn do we? we think we can love them to be the men we want,of course it must be us at fault.
No its not you are holding out in a very tough time and you can do it -daily 1 step at a time,a friend of mine a v glam ,intelligent woman showed me a picture of herself about 6 years ago, she was newly divorced with 2 young children -she looked so different and she explained that relationship was sucking her life away as soon as she got her around being a independant woman -she changed.
The penny dropped and things fitted into place,she is very content with her life and when her ex comes to pick the kids up she thinks of him as the kids dad but as someone she learnt a lesson from she couldnt imagine being married to him she did actually say even sitting on the same sofa as him turns her stomach!!
You will get there ,baby steps day by day but you can do it(hug4u).