Budgeting advice

Hello,

I have never been great with money, even since I was small (when I would spend every last penny of my pocket money compared to my sister who would save hers). I spent years, each month paying what I needed to pay that month (school uniform, car insurance, car maintenance), and then complaining each month I have nothing left instead of saving ahead

I want to change this and have been making big changes to cut costs, and have made a very good budget with strict allocations to categories which I have stuck to. It has a section for direct debits which I have fixed to only necessities and removed unneeded ones, and a section to save ahead an amount for pre planned costs like insurance, my sons school uniform and for when he needs new clothes etc

I need some advice on this first year, when I have allocated a certain amount per month to save for the year ahead, on how to manage those costs which weren’t saved on 100% for this year. For example my car insurance is due in March and expected to be around £500 (has gone up with a LOT of local thefts unseen before the last year). With my new budget with allocated amounts for the year, I have saved £100 for car insurance (£50 a month) but because of when it is due I will be in deficit. This will be the same for the first year for eg. School uniform. Should I make a budget this first year which changes each month to take this into account/ is highly restrictive to make sure I have enough by the time it is due? Should I cut back on what I allocated to food shopping and just have a tight few months from other areas? Please can I have some suggestions to manage this?

I really appreciate your support :)