I agree with the starch & steam the others have suggested. I've had a lot of luck with taming bias with steam, and I think this is where the problem starts.
I'm assuming your squares are cut on the straight of grain, which means once assembled, the bias in each square goes east-west and north-south across your quilt? Starching your fabrics heavily before cutting into them at all will help a lot, but you'll need to see if you can fix what you've got so far first.