The line of communication between a student and instructor is, in my humble opinion, the most important aspect of the relationship. I believe that a student's progress in a lesson is directly related to their ability to communicate with their instructor and vice versa. On the instructor's end, the ability to recognize how their knowledge is needed and clearly convey that information is what makes them a good teacher. On the students end, the ability to express their goals and speak up when they are feeling something (understanding, confusion, frustration, etc.) allows the pair to communicate with each other.
(on a side note, this concept of communication is also essential between horse and rider)
I have been a student and I have been a trainer.
I have been the one to hide things from my trainer, because I was afraid she would get mad at me or upset.
I have also been the trainer that has been on the other end, getting lied to (yes, failing to tell someone something on purpose is also lying!).
In my experience, most of the issues that have come between students and instructors have been related to poor communication on one end. Good horse trainers are not always good instructors, so explaining the way to ride through something will not always work for you. Remember that instructors do not fit every personality and you may not work well with someone that others find helpful.
When you do find a good instructor, that is capable of communication, don't spoil it on your end. Lying to your instructor about something, because you are afraid they will be mad at you, is not productive. It is your instructor's job to gather and process information for your benefit. If they do not have all of the facts, they cannot help you correctly. For example, if you gave your horse 10 days off because you wanted to take a last minute road trip, your horse will be the one suffering when you are afraid to speak up and say that maybe you shouldn't be working so much canter after he has been off for more than a week.
Maybe the situation is related to your instructor. Again, communication is the best answer. When you are annoyed with your instructor about something, not saying something will not only cause bigger problems. It will breed resentment in you, to the point where you will just get sick of it and maybe leave. Your trainer, meanwhile, might not have any idea that something was wrong. You are secretly furious and she is just going about her day, assuming you are happy.
Avoiding discussions when there is an issue with your instructor does not prevent hurt feelings. Sure, you might save your instructor from feeling angry at you, but you will do something even worse: You will make them feel like you don't feel comfortable talking with them. That you don't feel like you can communicate.
What I hear all the time from fellow instructors, after their clients have been hiding something is, "I wish they had said something sooner." The instructor feels like a failure, because instead of their students coming to them with a problem, they were the problem and their student felt they could not come to them for help.
In one case, a person I knew was so afraid of "hurting her instructor's feelings" that she simply took the horse from the barn on a weekend that the trainer was on vacation. The trainer returned to an empty stall, some flowers and a Dear John note that explained that she had decided to take the horse back.
I later found out why she decided to leave: "I loved working with my trainer, but she never gave lessons in the late evenings, she always left right after my lesson at 3pm."
"She was going home for dinner, so she could come back and teach me and any make-up lessons until 9:45pm," a fellow student replied.
The former student was so embarrassed by this and her sudden exit from the barn, that she was too ashamed to call her former trainer. The former trainer, to this day, talks about what a wonderful student she was and asks about her frequently.
In this case and others, you just have to take a leap of faith and trust that more communication will only help.