Your stutter is automated and habitual, body's default way of doing speech. To change this you have to recondition your mind to use a different pattern, almost like force yourself to write with the other hand, but harder. Every time you get stuck and push through a block, it gets reinforced. However, if you manage to deal with a block in a controlled fashion of your choice (virually any technique that is not based on avoidance, like replacing words), this new way of speaking will start building strength. Like a muscle, it has to be exercised frequently in order to make gains. At first, it feels awkward and even more exhausting (only mentally, not physically) but with continuous daily practice, speech will slowly become more and more effortless as the non-stuttering ways of speaking are much more efficient. I found this book to be a good starting point for self-therapy: https://stutteringtherapist.com/understanding-controlling-stuttering-book/