I'm pretty sure I'll never amount to anything
Wow. Why do you say this?
I'm just a worthless piece of shit
Sometimes we feel like we are worthless after such a bad time in our lives. Get yourself together again and be proud of who you are  because you tried your best. Your low self esteem will be the death of you after awhile if you don't have the guts to shape up.
Nah, I would never change.
Anyone can change. If you don't know where to start you can talk with a therapist and see what kind of state you are in. Do you have any goals you would like to accomplish?