If it's a concussion, you can visit the neurologist and check for neurogenic stuttering. If you only stutter on 'specific' letters, then it's not neurogenic (but I may be wrong here) and then it's very much psychological based. You say you only have the stutter shortly. You say you are a teen. According to the book 'easy stuttering' by Sheehan, 80% of kids who stutter, remove their stuttering. Your stuttering is not yet chronic. I recommend therapy 'if' it's chronic (after 5 years) because then you have build many secondary behaviors and iceberg. But right now, the odds that you remain a stutterer as a kid, are only 20% (according to research). There is no 'how' to remove stuttering. Kids will outgrow it automatically so build confidence to believe you can do it, don't be scared to do it and don't do rituals or anxiety-reducing activities to replace your correct habit.