This isn't a strategy post about how to get paid. (I've taught many a class on this!) This is a pre-strategy post, on how to get your mind right about this kind of situation. Because again: if you're regularly having these problems, you need to change things with your business infrastructure. I'd need to ask you a series of questions about your operations, in order to figure out where you're going wrong.
I am in dozens of hobbyist groups online where you inevitably see a regular trickle of questions like, "How much do you sell your __ for?" There's no shortage of advice given - in fact, usually there's a pile on in the comments, of all kinds of conflicting advice delivered by people who have never run a business, are helming one that is soon to fail, or who briefly tried for a brief while before bailing.