Possible meaning are 1) both instances of "burden" mean "message." Alternate translation: "You must no longer call your visions the 'burden of Yahweh' because they are really only every man's own words, and" or 2) the first 'burden' means "message" and the second means "heavy load." Alternate translation: "You must no longer talk about the 'message of Yahweh' because the words that each of you speak are what is a 'heavy load.' In this way"
To "pervert" something is to twist or distort it. Alternate translation: "you have changed the message of the living God, Yahweh of hosts, our God, to say what you want it to say and not what God said"