I, too, agree with just about everyone above re: the various pros and cons of the situation. I do believe you MUST be kind, and really, you must be honest. I'm about as "sensitive" a person as they come, yet, if a student of anything I have taught over the years came to me, privately and in earnest (with the best intentions, and no desire to hurt me) & said, " You have been exceedingly kind and supportive in our lessons. I appreciate that very much, and I really, really like you as a person. However, because I want badly to GROW and at this time do not have any issues with being worried to work harder/take on "scarier" challenges/fear being told right away when I do something incorrectly, I believe I may get that sort of guidance from Instructor B. Would you understand if I were to change over for a bit to her/him and see if that aspect of her/his teaching stays the same, and then keep you posted on how it's going? If it turns out this isn't a good match after all with "B", would you be open to my returning to you for lessons?"
I suggest you say it this way for a few reasons. I believe, knowing ONLY the facts you presented re: "A", that she/he has this teaching style for a reason. People, and especially kids of today's generation (absolutely NO OFFENSE to those younger than I here...I'm speaking mostly of little kids, 6-10 year old kids) are very often totally unable to process criticism of any kind.
They & their parents, sadly, (with parents being the primary reason so many ARE this way) have gotten used to the old, "You're perfect no matter WHAT you do!" & any trainer going against that has BEEN CONDITIONED to expect to get FIRED. With making money to live being paramount to just about anything else these days unfortunately, even riding instructors have changed from good and "growing/educating" people, to those who feel forced into giving "attaboy's" and as you said,"You're perfect! It's just the silly horse!" (when even you could later see, as a learner, that you had a correction you could have made but were not cued to do so).
My point here is to say they if "A" understands, after a good, honest communication session, that you are MORE THAN OKAY, and in fact WANT TRAINING, and not just ego-stroking from her, you MAY SEE that she is in fact a really kind, supportive, AND GROWTH-CAPABLE trainer! She may simply have been in fear of being literally fired for helping you to grow!
After you speak to "A", you may even want to have one final lesson with her and see if she doesn't just "turn into" exactly what you have wanted, and did get, with "B"-once YOU have given her the go ahead to get more challenging with you and more honest with you about where you may need to alter your riding. If that is the case, you will end up with the best of both worlds! "A" as your regular instructor, and "B" as your back-up when "A" is gone!
I think it's worth a try, honestly. Most trainers would find it very hard to sit on their hands/mouth and watch things done incorrectly, and not challenge their student UNLESS they have been conditioned to do so out of fear of losing clients for pushing spoiled kids "too hard"
I really hope you get what I'm "getting at"....and another thing you might do is tell her something like, "you know, if I do something that isn't correct or productive, if you could take me briefly 'back to basics', (as "B" did) & show me where my flaw began and then "school" it out of me, I would love that. I'd also love it if we could set a goal at the start & end of each lesson for me to work/focus on!" & see if she has the skill as an instructor to do so.
Perhaps she will pleasantly surprise you. Either way, you will never come out of the situation feeling like you either went slinking secretly to another trainer, or like you didn't give her EVERY chance to do what you need to grow, and you did all of that simply by staying open, honest, & "on the level" with her.
Good luck to you. These situations are so tough, but they
CAN work out really well sometimes!