Perhaps rather than “oh no, I can’t,” you can say to yourself, “I choose not to.”

I would like some new running shoes, but it is more important right now that I stay within my budget and demonstrate to myself that I am disciplined and not blown about by any want that pops into my head.
Running marathons is a big, expensive thing. This year I started a business, so for now, that’s the big expensive thing that’s taking up my time, energy, and my money. This is bringing me joy and it was a good choice.
I need shampoo, but I can probably last a few days until the end of the week and buy it with next week’s budget. It might seem silly to some people, but actually, I take pride in sticking with my budget.

Smile to yourself and rest easy in the knowledge that you don’t actually need these things.

Or, perhaps sometimes you can say: “Hey, some part of me seems to think I can’t afford shampoo. That’s interesting. It strikes me as perhaps slightly ridiculous. Let me sit down and talk to this money-frightened part of me and see what it’s really worried about, and whether those worries are real and reasonable, or just ingrained thought-patterns.”

Either can work. Good luck.