Hi Sunflowerpup
Are you getting any Child maintenance from your ex? mine is self employed and so I only get a little bit although he always seems to have new trainers, buys treats for kids etc. That is really hard.
In the first year of my ex leaving I spent a lot of time worrying about money and got as many of my housing/living costs down as much as possible. I switch my elec/gas every year and home insurance etc. having never had any savings with my ex I have managed to slowly pay off one of my overdrafts which has saved me money in the long term, its very small amounts but it all helps me feel in control.
I found a budgeting app which helps keep track of where my money was going and over time I have been able to see a gradual improvement.
good luck, it is really hard being on your own and wanting a better life for your family.