My sympathies, sounds like you've had a big lifestyle change (London to rural Norfolk very different) so it's not surprising it's taking you a while to adjust. I guess the loss of income is the root cause of this, sounds as though you are still a bit stuck with previous habits that were formed when you had a bit more disposable income? Although this board is really great for frugality tips and hints, what will really help in the long run is increasing your family income, do you and your DH have a plan for this - are promotions or pay increases at work possible (understand this may be slower than in London). Also as your DS is 3 (if you aren't planning more children), you only have a few more years to go of really heavy/expensive childcare responsibilities, you will presumably be able to take more on at work and/or reduce childcare bill when he starts school, so that's something to look forward to.
In the short term, I think you need to revisit your budget and how you allocate the small amount of discretionary 'spends' that you have left after all essential bills are paid. For me your DS's clothes should be an essential alongside utility bills etc. - although you can be pretty frugal in picking up stuff 2nd hand and from charity shops etc so it doesn't need to be a huge amount. I'd work out a reasonable annual spend on clothes (perhaps account for buying him new things 2 or 3 times a year to account for seasonality and growth, don't forget to budget for shoes and a winter coat as well and then divide that by 12 and put aside that money monthly. Don't do any impulse purchasing of clothes for him outside the planned shops unless it's a dire emergency).
I would dedicate some time mapping out and accounting for genuinely all essentials in your monthly budget - there are good tools/checklists to do this on money saving expert and money advice service if you google (I wouldn't count takeaways as essential but don't forget some other non obvious items like saving towards Christmas, insurances that have to be paid annually, TV license and any TV subscriptions, car repair/MOT fund, add up how much you need annually then divide by 12 and allocate this to your monthly budget). So many people think their problem is spending on non essentials/impulse spending but then when they add up everything they actually need to spend or are already committed to spending every month (e.g. phone contracts, car finance, loan repayments), they barely have enough as it is, so no wonder they go over every month - and then once you are already overspending it's quite tempting to go 'fuck it' and add luxuries/non essentials in as well.
Once you have done the full audit of your actual essential monthly spends, the amount you have left over each month is what you have as true discretionary spends. I would divide this amount into either 2 (half for you and half for your DH) or 3 (a third for you, a third for your DH and a third as a 'family fund') and somehow physically separate it from the rest of your money - either withdraw in cash at the start of the month (this can be good as you physically see the amount going down as you spend, although so much these days is online or card only cash is increasingly difficult to use) or move to a separate online account. There are good apps and tools like you need a budget that help you separate money in an account into 'pots' so you can see what is allocated to what, it helps you to see that actually even though you can have quite a large sum of money in the account, every penny of it is already 'spent'/allocated to expenses so none is available for luxuries.
You then need to be strict with yourself that any spending on things you have deemed non-essential comes out of your discretionary spends money, and this money only. If you don't have enough for something you want, then you don't get it. Cut up all credit cards and delete your Klarna. If you find it too tempting to have bills money 'sitting' in the essentials account waiting to be spent, then leave your debit card for this account at home when you go out so you can't get at the money (only take your personal spends money), or if online spending is more your poison, then perhaps switch the account to a brand new one so it isn't already set up on your online accounts (you can get good cashback deals for switching as well), and give your new card to your DH or a family member who is good with money to look after, or just put it into the freezer or something, so that you physically have to go and get the card to make the purchase, giving you time to pause and think whether this is truly an essential purchase rather than impulse purchasing?