My advice is not to blame yourself or obsessing over finding the root cause because STUTTERING IS NOT YOUR FAULT. When I was your age I used to not worry about my stutter because I thought I would just outgrow it with time but in reality it impacted every aspect of my life so I couldn’t really ignore it (hence it stayed until now that I am in my 20s). while it’s not your fault that you stutter, it is your responsibility to work on eliminating it if you want to. I urge you to read this book: “How to Stuttering and Love to Speak”. It is a neuroscience backed method to stop stuttering that I wish I had come across earlier in my life. I have just started reading so I can’t comment on the efficacy but apparently there are hundreds of success stories that the author describes in the book. All the best!