I've come to the conclusion that you may be confusing "has their own opinion" with "has their own truth."
2 plus two is 4, whether someone believes it's 5 or not. If they believe that it's 5, this is their (very confused) belief, but what in the world do we gain by saying that it's their truth? If you talk that way, you blur the useful distinction between being right and being wrong.
It gets worse. If I take you seriously, then I could respond by saying "well it may be your truth that everybody has their own truth, but it's my truth that they don't. And so if you want me to take you seriously, you've given me a perfect reason not to take you seriously.
Of course people have different beliefs. We usually take that to be a matter of people disagreeing. But if you and I genuinely disagree, and aren't just play-acting or using words for fun, we can't both be right. And if either or both of us is wrong, then at least one of us doesn't have the truth of the matter; we have a mistaken belief about the subject of our disagreement.
Now you might ask: who has the right to say who's right or wrong? But that's the wrong question. The world doesn't give a darn what I think about it. The world is the way it is whether I or anyone else know what that way is. Reality isn't up to us. It seems to please some people to think otherwise, but it pleases some people to pick the anchovies off their pizza; there's no accounting for tastes.
In asking the questions you've asked, you presuppose that there are correct answers. Otherwise, why bother? Why not just come to a belief and call it your truth? Asking a question seriously doesn't fit with thinking that "everyone has their own truth." Asking questions seriously supposes that there are answers, and that it's possible to get those answers wrong. Most of the time, that's what most of us think. It's hard to see why we should think otherwise.