@cherriesandapplesandberries - dh and I could have posted your OP, almost word for word when the dses were little. We'd start each month in the black, having paid off the overdraft from the previous month, and by the middle of the month, we'd be heading towards another overdraft.
What helped for us was setting out the budget - on pay day, dh would start with his take home pay (I didn't work), and would immediately subtract all the fixed outgoings - mortgage, energy bills, council tax etc - and what was left was divided up between the other outgoings - food, family fun, running the car etc.
We also kept a detailed record of everything we spent, so we knew exactly where the money was going - so we could see what was necessities, and where we could cut down on our spending. Each month dh would print out budget sheets for the different bits of non-fixed spending (food, car, his and my allowances, family fun etc), and every time one of us spent money from one of those budgets, we wrote it down and kept the receipt. It was tedious, but it really did work.
I started doing a detailed meal plan too, so that we did one big shop for the week, and had everything we needed in the house for the meals and the dses' packed lunches, and didn't need to nip back to the shops during the week - one thing that we noticed from keeping track of the spending was how going to the shop for a loaf of bread never ended up being just a loaf of bread - other things 'fell into' the basket on the way round, and we'd spend a fiver instead of a pound - and that adds up.
It wasn't easy, but we did manage to get control of the finances, and after a while we were able to stop recording all our spending in minute detail - but dh still has the monthly budget on an Excel spreadsheet, and we have internet banking (not even invented when the dses were little) so he can keep track of what's being spent, and knows when we might be straying towards overspending, so we can cut back.
Apologies if this seems like a lecture - I don't mean it to. I just know how frustrating and worrying it is when there is always too much month left at the end of the money, and I wanted to share with you what helped us get on top of it.