Why doesnt he do what he says.....?

My bf of a year and a half always doesnt do what he says. Btw Im 22 and hes 26. So everytime we fight he says things....and does the opposite. He makes decisions and tells me things about our relationship and then when we see each other its the opposite. Also he lies so much like he thinks Im stupid. He lies to make me think his life is so fab. When Im the one whos excelling in university and working etc and he flunked out etc. How can I trust what he tells me when he always ends up going back to his old predictable ways?