One of the most difficult things for the shy among us - and by "us", I mean me - is standing up for one's self.
There are any number of reasons why this becomes a thing, I suppose: fear of reprisals, figures of authority that treated any sort of attempt to defend or speak for one's self harshly, a crippling fear of being rejected for expressing an unwelcome thought or opinion, or simply being shy and wanting to "go along to get along". Or all of these.
But at some point - I suspect, as it seems to be coming true for me - one finds that one simply cannot continue to just "go along to get along". One begins to express opposition to being shut down, ignored, or - as the above picture suggests - "disrespected".
It is remarkable how swiftly things will change.
The first or second time that it happens, people will simply cock their head and look at you oddly. They may try to paper it over - "What I think you meant was X" - or simply just ignore what you have said and try to carry on. When you express your opinion or thoughts again, they may try the same thing.
When they realize you are serious, things become a bit more...difficult.
Somehow, they say, you have changed. You were not like this before - what happened to you? Why did suddenly become so wrong about things? Why are you being difficult about them - these are things that you had always done/gone along with/believed/allowed in the past - what changed now?
In other words, suddenly you have become "difficult".
It is difficult if you try this and you are an extrovert - but it is literally a living form of punishment if you are an introvert. Expressing yourself in public at all can be hard, let alone if you are trying to buck established patterns of behavior and interactions that have existed for years or even decades.
But even with your introverted fears, you quietly raise your wavering voice and express yourself.
Over time, your voice may become less wavering. I am not sure that your fears ever go away.