The tension is building up because you probably aren’t “block releasing“. You are just trying to push through the block. You can let go of tension by letting out a repetition of the first sound of the word, while simultaneously letting out all your air. Then take another breath and give the word another try. The tension that is hurting your chest is a result of trying to hide your stuttering. It is a secondary behavior that is learned. So you can unlearn it. I recommend watching this video to learn “block releases.” This guy is very encouraging and has a sense of humor too. He has other exercise videos on his channel that go more in depth to the “block release” technique, but either way, you will see it modeled in this video. Right when you begin to have tension, let out the stuttering block with a release in sound and air. And it will give a sense of dignity knowing that you are showing to the world openly that you stutter. The world expects people that stutter to speak with repetitions, so show them repetitions to prove to yourself that you are ok with people viewing you as a “stutterer,” even if you don’t choose to label yourself that way https://m.youtube.com/watch?v=UA4ZhaZ-zV8&t=253s