Hi SP,
I've struggled with assertion my whole life, and wish I could "win" or get the last word in with others, but isn't it draining?
I've been better when life was in balance. Another thing is to join a club which will sharpen your assertion skills, such as public speaking or a debating club. They're great skills to learn, so you can be clear without being negative, and transfer these skills to work and relationships, and it's fun!
No one has the right to be condescending, since everything is relative, and there are bigger more educated and better people whereever you are. Being civilized is about living with our diversities, and learning to assert yourself would be healthy for you, otherwise there's a wearing down of your dignity which which harm you.
With your artistic skill, would you ever, for example, criticize someone for painting an inferior flower? Of course not! But someone might criticize your art, if they were rude. To put that in context, Picasso, if he were living, could criticize your critical artist(the person who might criticize you), but who wants a world like that?
We can't choose our neighbours, workmates, in-laws, but need to become resilient, and a verbal skill program like the ones mentioned, might help. As artists, some of us just prefer non-verbal activities, and that leaves a skill untapped, and ourselves vulnerable...