You have to rein in the spending.
Lots of people think single parents are rolling in it on benefits. This is simply not true for anyone I've ever met or spoken too. I'm working and I still struggle.
You are probably used to a certain lifestyle. But you have to cut back. All luxuries can be cut back on (haircuts, clothes, makeup, booze, meals out, takeaways etc). You might not like to go without them but you can certainly live without them.
Check you are getting the best deal on your bills, switch to direct debit if you can. Get advise on any debts you have to try and reduce the interest payments etc. And budget like mad.
I have every single penny accounted for (excluding what ex pays for ds - that's for luxuries, fun stuff and treats and I never rely on it) on a spreedsheet and put money aside for bills, necessities, saving for Christmas etc. It's tough but if you are strict with yourself you can do it.
You have to seriously look at what you can and can't afford. What really matters, what is vital, which you'd like to have but can do without, and what really can go really easily.
When I was living on benefits (couldn't afford to heat rooms much) we slept in one room together with jumpers and tracksuit trousers on over pjs. We would sit on the sofa with a duvet over us to keep warm and hot water bottles.
You can cut back on food spending too, buy economy and own brand items. If you run out of food money then you feed the kids and go without yourself. Simple as that.
If you keep spending beyond your means you will end up not able to pay for food, rent, water and electric. Those things matter, internet, starbucks, and nice clothes (as examples) don't. Even things like a car and a tv are not strictly necessities if you can't afford them.