STOP

If you're self-employed, stop spending your gross income.

The biggest cause of business failure isn't lack of sales, but that surprise tax bill that kills cash flow.
Build good habits. 

Every single time a client pays an invoice, immediately move a percentage of that money into a separate savings account. 

Do not touch it. Do not borrow from it. In fact do not look at it, pretend it doesn't exist.
"That money is not yours", It belongs to HMRC; you are just holding it for them.

When your Self Assessment bill lands, you won't panic, just make the transfer and remove all the stress from paying your tax bill.