1.5.1 crashing while loading a server or single player world (any game mode)

When I tried to open a singleplayer world, the game tried to load it, but after "Loading World, Building Terrain" the screen quickly snaps to the main menu, and then to a dirt background and crashes. This also happened when I tried to join multiplayer servers, after "Logging in," the screen changed to dirt and crashed. This happens every time, I've tried so much to fix this; from force updating to deleting .mincraft folder to downloading java 7. This crashing bug has been going on since 13w09a but I was never able to find a fix. I haven't been able to play any minecraft since 13w09a and Its really annoying. Please help. (P.S. Im not sure if this bug is just limited to Macs)