If you've outgrown what the barn & trainer has to offer, she should understand. I've had several students over the years do that when they decided to go into a discipline that wasn't one I was involved in. I supported them in those decisions and even went to watch & cheer them on at shows under their new trainers.
Just be honest about it. If she doesn't understand the reasoning then there isn't much you can do to make it any easier. Be sure to thank her for all that she has taught you so far, for giving you a good foundation to build on, etc. Try to make it a positive split.
Life is like a camera. Focus on what's important, Capture the good times, Develop from the negatives and if things don't work out, Take another shot.