"I know how you feel."

No, actually, I have no clue at all how you feel.

I have no idea what it is like to be you or to understand precisely how you feel.

I have never experienced anything quite the same as you have.

Telling you that "I know how you feel" is similar to say that I know how a bird feels when it is flying in the air (that is, after I've flown in a plane.)

But do I really know how a bird experiences flying?  Not at all.  I can imagine what it might be like and how it must feel, but I can never truly understand what it is like to be a bird.

Why?  Well, because of something called a "consciousness."  Surprisingly, even with advanced modern science, humans have no way to measure a conscious state of mind.  Sure, I can understand how someone else's body works in a physical manner.  I can even understand what part of the mind experiences pain.  But I cannot experience that persons pain and react the exact same way, in regards to consciousness, as they do.