You may not be up for this (which is OK), and others might disagree, but I would consider being perfectly honest with the trainer you are leaving. You could simply say that you feel like your training has plateaued and that you tried out a place closer to your college that seems to be helping you progress beyond 2 ft singles. Thank the coach for her time and everything she has taught you, and then don't look back.
I feel frustrated FOR you with coaches that fail to help their students progress, especially as the coach won't tell the student that it is time for the student to move on to a more advanced coach. Coaches need to hear from their students if/when they need to improve their coaching skills - or they will continue to loose dedicated students.