All of this I wish I had cottoned on to earlier.
Budget, and don't spend more than you need to, but still enjoy yourself and don't miss out on having a laugh or doing things you will remember forever.
FIRE doesn't make sense if you're wasting nor not almost making the most of your best years.
Don't go shopping when you're hungry.
If you lend people stuff, write it down.
Investing in learning, especially something which has a demand and gets compensated accordingly, will be the best investment you ever make.
Pick a career in something which has a future, but also don't be afraid to switch careers if you can see yours having a dead end.
Ideally work for yourself as soon as you can, if you're cut out for it. Make profits from yourself, don't let some other clown take them from you. If you do work for someone else, don't be afraid to ask for more money each year, if they're getting more out of you each year. If your pay matches inflation, then your boss is getting a good deal, as he's getting a more experienced version of you for the same cost he got the inexperienced version.
Invest in something diversified, as soon as possible, every £1 you put in will be worth 3 to 5x after 2% inflation in 20 years or 10-20 x after inflation after 40 years.
Still go on your nights out, but spending £30 on a night out, rather than £50, will save you £200-£400 for in 40 years time, this scales accordingly. Buy the £600 phone, rather than the £1000 phone, and invest the difference, if you have it. If you don't have the difference, then don't buy the more expensive item, you don't need that version and the ROI is terrible.
The hours between midnight and 4am are an expensive time to be drinking, as you drink more expensive drinks, more of them, and you won't remember anyway, and you'll feel worse the next day. Girls you meet between midnight and 4am might be fun for half an hour, but you'll probably not want to be with them long term!
Take debt/ loans when it's <5% even if you can afford to pay outright or pay it off sooner, as long as you can actually afford what you're buying with some left over, as the return on you investing the difference instead will save/ make you money.
Don't take debt more than 5% if you can avoid it, and certainly try and avoid credit card interest, so don't invest with a 10% return, when you're shelling out 10-18% interest on a car or CC, just pay the car or CC off first.
Taking 3-5% mortgages over longer terms, to free up more money to pay off car loans and CC's faster will save you a fortune.
If property prices are not booming, or not expected to boom, and if you can rent or house share cheaply, just rent/ house share, and don't buy, and invest the difference. There's no point taking on a 5% mortgage if house values are not going up 5% a year and if you can rent much cheaper. Use the opportunity that you're renting to move around where the money is and get up the promotion ladder by moving jobs faster if possible, but not too much where you're seen to be disloyal. The time saved through promotions and earning more money quicker will make things so much easier long term. The money saved by renting will buy you the house outright in 20-30 years and the extra money you make by climbing the ladder faster will get you a better house or more holidays.
You won't retain all the friends you have in your teens, 20's, 30's, 40's even if you stick around in the same place, but no matter where you are or what you do, you'll make new friends along the way, they might even be "better" than your old mates.