Welcome to the Forum!!
From my perspective, why should you stay?
Your trainer is not
interested in the discipline you chose to ride and want to compete in.
You pay exorbitant board for a barn falling apart and with no amenities.
To ride you must pay another large fee and walk 15 minutes either direction to reach it..
Your trainer doesn't go to shows of what you want to show at...
You're stagnating where you are.. I'm not seeing any reason to stay...
If your trainer does not meet your needs, then you must move on.
If the barn not meet your needs, then you must move on.
Can't see the trainer being upset or offended that you need to move barns and instruction when what she offers is not what you wish to do, what she is instructing in nor has facilities in which your horse can ride and practice in.
There should be no hard-feelings....not that I can see.
You have outgrown what she is willing to offer and the direction you want to pursue is not what she teaches.
Appreciate all she has given you, but time to go is at hand....
Go look far and carefully for what you think you want before outing yourself first.
Find a barn and take some lessons under their instructors a few times first, get in and watch the atmosphere because "show" barns, competition barns is a very different place than what you have experienced and none are the same in atmosphere, cliques of riders/boarders, actual costs to be a boarder/student and requirements of when you show...make very sure where you head to fits your plan and your expense account cause it does get pricey real fast.
Good instruction and boarding barns easily cost what you pay now for board, add in the "show" caliber and it just went up substantially for the student wanting that.
Look around, take notes and compare and remember it is also about the care of the animals and what is dictated by the barn in farrier, vet, chiro, lessons, clinics, required grooming, tack, clothing...oh the list is extensive when you go to what you dream of...
Good luck, but no...staying at this barn is not in your best interest. It gave you a launching pad, now you need to find the wings to soar with that basic education acquired.