You were kinda right, but it really is about the 2 sides of struggling: You choose the dark way; cutting. Or you choose the bright way; Writing poetry, lyrics and stuff to get things of your mind, and focus only on the thougths of what is written on the paper, is written by your inner soul, which has been buried inside. (Much like Andy did, and it helped him.) This was self-stated by Andy :)
But on the other hand, The meaning of Knives And Pens is different for everybody.