It's good to get these things off your chest. I feel it frees you in some ways and you don't have as much of the crushing weight on your shoulders, It also seems likes it been building up. You have every right to feel the way you are currently feeling. In my most anxious times, my stutter also went to shit. Now BREATHE, I would start by letting your teachers and your school faculty know about your speech impediment and try to see what they can do to accommodate. You're currently under a lot of pressure and your anxiety is heightened as a result of this but don't worry things to get easier. Your dream is to speak fluently but in order to do that, you have to keep practicing. I know it sucks to listen to your recorded voice but you have to do it. There many other techniques I learned over the years that I learned in speech therapy such as slow and easy speech and breathing in between words. Also, It's pretty good that your realizing stuttering words and then replacing them with words you can say. You do have to keep talking in order to increase your fluency. What helped me was thinking about what I was going to say(don't overthink it) take a sharp breath in with the nose and on the exhale start your sentence. Remember it's ok to have blocks and stutter on words. One of my favorite quotes that I saw was by Charles Van Riper to the question What is stuttering."Stuttering is everything we do trying not to stutter.". I had many times when I stutter and instead of complete my sentence I would shut off and not do it or look at the ground cause I didn't want to look at people's faces while I was stuttering and that empty expression of hurry up and finish your sentence. I overcame my stutter a couple of years ago, not fully as sometimes it still comes back but achieved proficiency. Your still young and with practice and seeing what works and what doesn't. I think you can also achieve your dream