''I've lost my cat.'' This is your only correct answer. If you say ''I lost my cat'', you must tell us when you lost it. ''I'm lost my cat'' is broken English. It means YOU are lost, and your cat doesn't even fit in the sentence.

I lost my cat or I have lost my cat :)

"I lost my cat" or " I've lost my cat"

”I lost my cat“ is right

"I lost my cat." is correct...It means you do not know where your cat is.(you can't find your cat) But the (incorrect)second sentence makes me think that maybe(???)you are trying to say " I'm lost without my cat." It would mean that you don't know what you're going to do without your cat in your life.(because you love your cat) :) But I hope you didn't really lose your cat. :( ...........