Here's a little factoid that keeps getting reinforced through the experience of life:
Healthy people don't have a problem with your personal boundaries. Most of the time you don't even have to assert boundaries with a healthy person because there is mutual respect, and when you do there is quick correction. But with an unhealthy person, your boundaries are a threat, an offense, and an accusation all rolled into one.
I know there are books out there about how to assert personal boundaries, and that's fine. But what should be emphasized more is that you only have to work hard at maintaining personal boundaries with people who are unhealthy. Why?
Because they don't like your boundaries, and they are probably used to getting people to back down with them. When you maintain your individual integrity, limits, and values, it set limits on them getting what they want from you. It doesn't allow them to have everything on their terms. It may also highlight that they are doing wrong, or make them feel like they are doing wrong, and they don't want to look at that. They may try to make you feel badly, guilty even, like you are doing wrong. They may use veiled or explicit threats to get you to back down (I'll hurt you, I'll leave you, I'll replace you). They may get angry. They may entice you, lull you with flattery and approval. And they may hound you with a sense of obligation to give them what they want.
What should also be emphasized more is that learning to assert your limits and values in a relationship doesn't "fix" the other person. They are for you, not for them. Learning to better assert your limits and values is something that becomes more important with difficult and unhealthy people, while we can relax and be a bit more flexible with people with whom we share mutual respect and care.
So, that begs the question... if you have no real obligation to spend energy dealing with the unhealthy person, why do it? In many cases, our boundary ought to simply be "No, I'm not dealing with you anymore, not like this."
But often we can't. We won't let ourselves. We want to prevent some terrible outcome that we fear. We don't want to feel like we've failed. We don't want to be rejected. We like feeling appreciated too much. We like feeling like the hero. And so we remain pawns for people who will look for ways to exploit every crack and pinhole.
Yes, we are all unhealthy to some degree. We are all difficult to some degree. This can keep us humble when dealing with difficult people. But there is a difference here that doesn't really need explanation, and discerning that difference isn't wrong, judgmental, or mean. It is wise.