How can you deal with people who say they know how you feel better than you do?

You tell them politely that they don't know how you feel because they are not you and they don't live in your shoes. Your situation is different regardless if the same event happened ( like say both you have a pet pass away, it might be more tragic for once person versus the other given the circumstances. Like one person's pet might have had a natural death and the other might have been run over.) Regardless, only you know how you truly feel and no one else knows that better than yourself because well you are you.

The best thing to say is something along the lines of, " I'm sorry but you are not me and therefore you don't know how I feel better than me because you don't live my life."

If you get the point across that this other person is not you and doesn't live your life they usually will come to an understanding and they will get that they can't understand you better than yourself.

Hope that helps.