Isn’t this just the natural response to having had to have strict limits before? You’ve written about the times when you were quite poor and couldn’t afford things and you’ve written about the debt you’re currently working your way out of. That feeling of stress/panic/what have you that lacking money can bring never quite goes away, even when the money situation is light years better. So even if you could afford it, the lurking sense of fear (probably closest feeling) means that if you know you don’t need it, it becomes harder to justify. You have to balance the joy of the treat with the potential stress it may cause, no matter how small the treat.