I believe that thicker skin, comes from a tremendous amount of self acceptance. For instance, your first post alludes to an implied sense of having to take what you can get, in friends, because you 'have schizophrenia' . In that mindset, it's more like schizophrenia has you, not that this is you, someone who just so happens to have schizophrenia.

Being assetive, isn't about being aggressive. It does involve a degree of confrontation, or even something as simple as, asking for clarification. (Statements such as, 'what do you mean, by that? ' could be coupled with, I don't want any misunderstanding, could you elaborate) those are assertive, polite, yet gets you to behave more directly, without having to grow resentment nor wonderment.