How do I get my old passport back?
I dont think thats possible. When you apply for a new passport you need to specifically mention that you want to keep the old one. It will be made invalid then, usually by punching holes in it, and when you go collect your new passport you can take the old invalid one with you. When such a wish is not made clear, the old passport will simply be destroyed. So I fear your old one has gone through the shredder and cannot be recovered.