In your letter, you say loud and clear that you know your boyfriend will never change. So, the question here is, why do you want to try to make things work when you know they never will?
It sounds like you are in a conflict between your head and your heart. Your head knows this man is beyond repair, but your heart keeps wishing that you can change him.
I cannot tell you which path to choose. All I can do is start you thinking about what part of your personal history is being played out in this impossible rescue fantasy. I suspect that you were raised by a parent that was impossible to change, but, being human, you kept on hoping that one day he or she would change. It sounds like your hope for the happy ending to your earlier trauma is being transferred onto your relationship with your boyfriend.
It would be good for you to find out what piece of your history your mind hopes to heal through this relationship. Then, I think you need to remind yourself that you can't force a leopard to change it's spots. You can sure try, as I know you are tempted to do; but, beware: You are in for a lifetime of misery and failure.
I hope that understanding the link between this current relationship and your early history will deliver you from this hopeless plan to change a confirmed criminal.
Let me know how you do.