Advice/Help needeed

Advice/Help needeed Hello guys 23(M) here. Been stuttering since childhood. I recently started visiting a therapist, she told me I have severe cluttering problem which has developed into stuttering. I don't know how I feel about that diagnosis. She told me to practice in mirror but whenever i try to speak the difficult words I get gag reflex? Why is that happening and how to stop that. Also how do I practice speaking a word that I know I will stutter on but can't find a replacement for since its a technical term and I have to say that words in almost all the interviews I will give.