Ok so i solved it. After trying everything else I hooked my tv back on, started the game (with the black borders still on the sides), changed the resolution to 1360x768 (which gave me a full screen again). I closed the game and hooked my monitors back in and started the game, the borders were gone so i changed the resolution to 1440x900 and voilá it worked.
Wierd solution but there it is.