If your lisp were self-correctable, I would guess that would have happened by now. The guide that you need is a skilled speech/language pathologist. This is the professional who is qualified by training and experience to provide skilled analysis, rehearsal and feedback in order to modify an established speech pattern.
Your desire for change, and commitment to practice what you are learning, can speed up your project.
Many speech/language pathologists can help you to reduce your accent. I would choose one whose accent I liked!
Ruth Alice Jurey, M.S.