-let me start by saying sorry if this is in the wrong place

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Hi, my county's solo and ensemble is coming up in about a month, and my private lessons teacher has assigned me on the triple tounging sections of carnival of vinice from the arban book. Before now, we've worked on triple tounging, and i know how to do it, and i know all of the "theory" behind it, but i'm having a big problem. When i go for the middle tongue stroke, the "k" tongue, as most call it, is getting very throaty. it gets a gagging noise to it, almost like a "gyeuh" sound before the note, which i realise is because my tongue is back much too far, which causes my tounge to move to much, and will overall restrict my speed. I know the correct syllable to use when I play, but for some reason, when i get a embechure set and my air moving, it either goes back to the same position as it was, or the syllable doesn't come out right. Is there any advice on keeping the stroke in the middle of my tongue anyone could give me? Any kind of secret, excersizes, website, etc that could help me with my problem? Thank you for your time
-Mt2