Even I started to stutter at 12-13 years of age like you. If your abdomen is tight, it means you didn't detect your smaller stutters that came before the main stutter and didn't notice that you ran out of breath. Just breathing in will help you a lot and it will help reset your stutter as well. Off late, even I have been stuttering with myself a lot which I never experienced before. I feel it is because I haven't been social and outgoing enough off late as I spend so much time indoors by myself. For most people who don't have a comfort zone where they talk smoothly, I would really recommend reading out loud as getting used to being fluent really helps to ensure that your body is capable of speaking fluently and it ensures that you just have to deal with stuttering alone. I would also recommend what I call the crystal method. Crystals are formed through a process called crystallization where it starts from a small point and it expands.and gets bigger. You need to start your fluency somewhere and for most people, it is with themselves. Then, you have to gradually increase your crystal size to include your family members or shopkeepers or friends, classmates, teachers, restaurants, strangers, etc.