An infinite progress

[Spongebob]

No, that doesn't satisfy the question.  I asked had you ever felt something and later found out it was NOT true, meaning it was FALSE when you felt it?  IOW, you "knew" something to be true when it was actually false.  I can give you an example if that helps you understand the question.

Let me put it this way.......my official diagnosis (in real life), is bipolar disorder, because that's the closest thing to the truth, according to my psychiatrist. Now, if my psychiatrist decided schizophrenia was a more fitting diagnosis, then that would be the closest thing to the truth. So truth is an evolving thing. What is true today, might not be true tomorrow.

Okay.  I'll assume you are being honest here and will treat this revelation with all due dignity.  Either diagnosis is extremely serious and deserves the utmost respect.  I know a little about this sort of thing due to some family connections, enough to know that such diagnoses can be very difficult at times and in some cases almost impossible.  So that makes treatment even more difficult.  I say this because this is certainly not a good example of believing or knowing something.  For one thing, you aren't the one making the diagnosis, so you are better served by accepting a doctor's medical opinion.  And, as demonstrated, such determinations are not easy even for highly trained physicians.  So knowing this sort of thing with certainly isn't always an easy thing.

A better example, though a bit flighty, is the Mandela effect.  This is a very common condition where people believe they know something from their past but it is actually false.  It can be disconcerting because often times they have very vivid memories of this topic and there's a bit of a conspiracy theory that someone has been tinkering with the timeline.  A few examples:

Darth Vader saying "Luke, I am your father..."  He never said that.  Vader actually said, "No, I am your father," in response to Luke.

From Snow White, the line "Mirror, mirror on the wall" was actually "Magic mirror on the wall."

The children's book series Berenstain Bears is often remembered as Berenstein Bears (notice the spelling).  I fell for this one myself.

And the best example of all, the condition is named after Nelson Mandela because many people remember him dying in a South African prison in the 1980's.  In fact, he did die in the 80's but was released from prison and went on to become SA's president in the 90's and lived until 2013.

So, as I said, these are somewhat funny examples but still very clear evidence that people often don't know what they believe they know.  They believe they know something, perhaps they feel they know something, but it's clear that believing and feeling are not the same as knowing unless you define "knowing" as simply believing in something very strongly.