You know you are capable of making those sounds fluently in the right circumstance. But you expect you're going to stutter, so you are more tense, so you are more likely to stutter. Try thinking, "I'm going to say it. I might stutter, I might not. But in one minutes time I will have said it, and it won't really matter that I stuttered" the more you can be ok with it the less tense you'll be and less you'll stutter. That's how I overcame my real hatred of takeaway drive throughs. I just thought to myself. In a couple of minutes I'll have what I want and what happened in between doesn't really matter. I know it's really hard at first especially at your age but acceptance has been my greatest step in fluency.