Sorry…I haven’t read all the responses…but right off the bat, based on your description…honestly, this horse does NOT sound like a great match for you OP.
You are 5’4". He is 17.3. That ALONE makes it not a great match. I’m 5’9". I have a WB/TB cross gelding who I’ve owned since he was 2. He is 17.1 and too much horse and too big for me. I started him, taught him how to jump, rode him in his first shows. Got him to training level eventing (and jumping more than 3’6" courses) but now do not ride him. Why…because he is too damn big and strong for me.
I have a pro riding him now…and they are aiming for Rio…and have a shot at it. People see him and ask why I’m not riding him. And my answer is that he is NOT my sort of ride. It is NOT that I am not a good enough rider for this horse…but I know we are not the best match. How do I know this…because I have and had horses who ARE a good match for me and know what that feels like. I love this horse though…and will not sell him (although we are syndicating him–he will not ever be for sale).
You have to take emotion OUT of the picture. I have 15 horses…I CAN ride them all. I CAN ride green and I can ride strong. But I KNOW what is a better match for my riding. That is two of my OTTBs. One is 16.1 hands and is like riding a sports car and the other is a bigger 16.2 hand mare and also more of a finesse ride. But both are brave and forward. THAT is my sort of ride. I’ve re-started them both from racing (I like green horses). I do not ride overly strong horse as well (my big horse).
So besides being a match for you physically…they need to be a match for your riding. And just because you CAN ride them doesn’t make them the best match for you as a rider. All I hear is your trainer being honest with you…and this is supported by the fact that you still need to lunge the horse before your lessons, given his size, training and other issues…he really is probably NOT the best match.
What you want to do with that is your choice…you can keep working at it–but it will not change the physcial match issues. You can learn a lot from this horse but if he is a potential GP horse, then now (before he gets much older) is the time to sell. But if you do decide to keep him…then you will need to work hard with your trainer to keep improving. If you do that…just be HONEST with yourself as to the issues. It isn’t all about the horse…or all about the rider…having the right match is important. And while you can learn a ton from EVERY horse…when you have that one (like my two OTTBs) that just suit you in all ways (not because they are push button but because you both are the right match for each other) it is VERY cool and more fun than anything.