I have almost the exact same form of stuttering. Like 96% of what you just described match mine except my stutter has never disappeared as such. I feel like I've been better equipped to mask it from time to time. I'm 22F. Now, I'm not an expert but being a stutterer myself and having pursued treatment from speech therapists and psychologists, nothing clinical has worked for me. But as with people having anxiety, support systems have done a great deal in making me braver and more secure. At the end you asked if it's insecurity, I think yes. Stutterers have had experiences where they've been made to feel lesser for their difference in speech and that naturally invokes insecurity. But it can be lessened with support systems. You have to start believing that it's not you that's the issue and that's the most difficult task. I think in the long run, you will have fluctuations, ups and downs and be better at controlling your blocks but I can't say anything about whether it can be cured. From my medical reading, there isn't really any cure yet (except if I'm missing out on any study, in that case, I'm not sure).