There certainly is a such a thing as too much horse show time. The show season can be extremely stressful to your horse. Between the change in location, footing, turnout, water, company, noise, trailering and the hard work you will demand of him it is not too smart to just show and show til there is nowhere to go.
There is also such a thing as too much schooling time. It is not just about how often you school though regularity is important. It is about how wisely you use the time.
As a dressage clinician I see many good hearted and well meaning riders that arrive at the clinic expecting some sort of miracle. Generally we ( my husband Paul is also a Grand Prix dressage trainer), do manage to give every participant at least one 'ah ha' moment. When you school your horse, every time you ride him he should have at least one 'ah ah' moment too.
This won't happen unless you formulate a plan. For example, why show up at a lesson and say, " I want to do a leg yield," when you have not educated yourself about the movement. What is it's purpose? How do you accomplish it? What pre-requisites are needed?
I actually had this happen one time. A lady who was unable to have her horse come close to being on the bit and with whom we had been working just a couple of times, made this announcement on our arrival at her farm. Paul and I just looked at each other. What?
So Paul, being the more diplomatic of the two of us suggested she give it a go. Hoping he would then be able to better explain why she did not have the pre-requisites in place i.e. the scale of progression and where she trotted about on it! Needless to say it was a disaster. No. She did not know the aids or have any notion of when and when not it should be used. Never mind any notion of why this non-classical movement is not to be overly used in training the horse. Sadly, she did not seem interested in learning either.
There are other riders that do zero homework between lessons with their trainers. What is the point of that? The trainer ( hopefully) will give you a couple of exercises to work on during your solo rides. The idea being you take what they have taught you and practice it for review next visit.
So there are clearly many riders who do not formulate any kind of strategy for progression. Sometimes I am asked by students where they should focus and what can they do to reach say 3rd level by the end of the summer and where and when should they show to garner scores. I am very happy to oblige with answers and direction BUT if they don't do any homework between my visits they will simply fall short and have a bad go in the show.
Their horse will be 'overasked' and confused and they will perhaps feel a little humiliated. Though all too often they will blame everyone but themselves for their shortcomings.
So find a good trainer, work with them regularly. Map out a plan and do your homework.
There is so much more to riding than just riding. You must bring a good frame of mind, an educated mind on the task ahead from reading and reviewing videos ( personally books are still the best for this in my opinion) and a desire to improve and work hard.
I urge you not to expect more of your horse than you do of yourself. There is 'No 'I' in team' after all. And you and your horse should be a team with you handling 51% of the directorial duties.
To paraphrase something Denny Emerson once said to me, " You'll find in teaching (as you age) that you only wish to teach those that want it as much for themselves as you want it for them." I agree. I don't want to waste my time, or theirs or their money.
I do care about the success of the student and the well being of their horse. A lot. So please folks. Map out a plan and get busy.