This is unfortunately nothing new and is just one of the instances where stuttering is different for most people. For me personally, I „feel“ the words in my mind where I will get stuck, and singing in my mind doesn’t change that. What helped me, personally, was speech therapy, constant practice and lots of time. The difference in my stuttering from when I was a young teen to now (age 27) is huge. Lots of young people here are looking for a cure, but the only consistent thing between each technique and therapy is that the techniques take time to stick.