I have to get a car for work unfortunately.
Are your miles expensed, or is this purely for commuting?
If the latter - why spend £4.2k a year x at least 3 years just to stay on the road? You can still buy a decent reliable older eg hatchback for less than £4k.
If you don't have that ready money, it might be worth borrowing it. Even with interest, if you fix a good rate & allow for a couple of hundred to pay a TRUSTED mechanic to vet your potential purchases, you'd be £7k - £8k better off over a 3 year period.
If the former - how many miles, can you claim the full 45p per, & does this exceed your proposed £350/month outlay - every month?
How much are you going to lose when you drive it off the forecourt?
How much will it depreciate over the 3, 4 or 5 year term?
What does the fine print say about how much you can be stung for at contract end, who assigns the Final Purchase Value (ie potentially rips you off on a buy-back or exchange) & what additional fees are there if you have exceeded an agreed mileage?
For non-car related stuff, start writing down your expenditure. PP have suggested apps or a spreadsheet - all good, & you may find a notebook works for you just as well. It lends immediacy & instant accountability. My sister used to do this when she first left home - literally, if she spent 15p on a biro, she wrote it in her account book. It turned her into a SAVER! A tenner a month here & there into various pots, & throughout her 30's & 40's she had several little nest eggs maturing, which she either reinvested or spent on fab holidays.
I taught my girl to do this when she was 14 & experiencing Inevitable Teen Anguish that she could not buy all the things ... it's simple maths, it just takes a bit of application & consistency.
Start with the balance of what you have in your account, underneath it write the cost of each item/service/utility on a separate line. Next to that cost, on the same line, subtract the sum spent & write the new balance.
2 months of this & you can assess where it's all going & start budgeting instead of feeling you have lost control & money is floating away from you.
Boring as hell for a while, but the trick to any chore you dislike is to get quite good at it. We tend not to dislike doing things we are good at.