I believe decrease in shame relates to desensitization. You can desensitize yourself mostly by being open about what you're ashamed of, not letting it stop you in life, sooner or later it should become easier. I think shame is not only about stuttering in general but also about us knowing how it limits us in life. If you don't let it limit you and still do everything you want despite it you usually have less things to be ashamed of or dissatisfied with. At least it fits for me and I notice it in other people who don't have problem with their stuttering. Shame is one of the worst feelings to endure, sadly, in order to minimize it firstly you need to accept your stutter and that sometimes you're gonna stutter. Getting rid of perfectionism is also very important. But I understand you and share the same feelings as you, my stutter also intensified with years, it's hard to be okay about it.