Repeat until you believe it…
“My partner and I are getting married and the three of us are going to start a new life closer to his family.”
It’s hard, but I agree, it sounds like you’ve come to the end of the road with this trainer. You could turn the thought process around a little and think… She doesn’t want me to make progress too fast because then I would outgrow her capabilities and I would look for a new trainer anyhow. By keeping me low and slow, she’s keeping the paycheck.
But in reality, she’s burnt the bridge and will now lose the paycheck.
When you are ready to give your notice, make sure you have a place for him to go (and a ride), because, as seen on other threads, this could get ugly. Bring her a nice thank-you gift, say all the nice things - thanks for finding this wonderful horse, thanks for all your help over these past X years, thanks for the great care, thanks for being a friend, yada yada yada, but partner and I are moving to CT to be closer to family and Horsie will be leaving on X date.
Be prepared to pay her an extra month board if you need to leave quickly.