For those who are self-employed as sole traders:
The challenge is not to spend money that isn't really yours. Sole traders receive gross sums that include tax, VAT (if their turnover is over threshold) and which also includes regular business expenses that have to be paid promptly like premises rent, accountant fees. I'm professionally regulated so also have to pay for practising authorisation and insurance.
I got into a lot of debt early on as all this money went into one account and I lost track. I paid off my debt and now have the following system to hive off business expenses:
All money goes first into a business account
I have a monthly standing order to a ledger account for tax/self-assessment - this means come Jan and July I can always pay my tax
I have another regular standing order to a ledger account for regular business expenses (professional indemnity insurance, authorisation to practice, accountant fees, premises etc). This means every year I have the lump sums I need to meet basic regulatory requirements.
I also pay myself a set "salary" every month to my personal account that covers all my essential personal expenditure, including savings (this was worked out after budgeting my household stuff and ditching anything we didn't need). I never use the business account for personal purchases, ever.
Anything else just stays in the business account. Generally there is enough there to cover me for 4-6 months if I can't work and also to pay VAT every quarter (though really I should have a separate account for this too)
I've done this for years having previously got into a lot of debt which took a lot of work to clear. This means I am always compliant with my basic professional obligations, never behind on tax, and - the important bit don't spend what isn't really mine.
It was much harder to do this before I was a high earner, but at least it made it much easier to work out what I was in debt for - I was spending stupidly on personal stuff whereas I should have prioritised business non-negotiables like tax and cut my cloth accordingly.