@Anonymous, I love the idea of "releasing" or "letting go" a client who might not be a good fit for you so you can both move on to more successful professional relationships.
You also bring up a really good point about mindset. Self-employment offers the opportunity to choose the people we prefer to work with, and that is very empowering. On the other hand, that power can be difficult to leverage for those who are just starting out. For me, it was a big deal to reach the point in my career where I wasn't afraid to turn down work, knowing that there would be another client or project not far down the road.
I guess what I'm saying is, it's one thing to get to that place in your business journey; it's another thing to feel comfortable there!