You made a mistake, that's why you can't do anything.
p^2-2p-pt+2t does not equal (p^2-pt)-(2p+2t). If you pull the minus sign out of (2p+2t), then it should be (p^2-pt)-(2p-2t). The minus is attached to the 2p but not the 2t. The 2t is plus so if you pull the minus sign out, it should be -2t.
