OP, I'm glad you have a trainer helping you. I'm not going to say that a teenager can't train a horse because I am watching one do it right now, but she is definitely one of the exceptions. I work with teenagers all day, every day. I know what they are capable of. I have some serious cowgirls in my classes; girls who have been raised around horses, who have been barrel racing since they were 4 and who's parents train horses for a living, and most of them are not capable of the specialized training that your horse needs.
The teenager I mentioned at the beginning is one of the most knowlegable horse-women I've met. She is currently training her 5 year old Paint gelding. She's been riding him for a year, but she's still training (and probably will be for his life)...The key is that she trains WITH a trainer. She is riding the horse, she is doing the work, yet she has a professional (who by the way, seeks help from other professionals all the time) guiding her. I think a true horse trainer is never done learning, just as every ride is essentially a training session.
A teenager who admittedly doesn't have a ton of experience should NOT be discounted as capable of training/retraining a horse, but that same teenager should never think that they can do it themselves, or that the job is ever finished. I train dogs. My dad trains dogs and has all of my life. When I was faced with a particularly challenging dog, I in no way thought I could do it myself. I did come to a forum like this for suggestions, and I had a professional trainer friend on speed dial and spoke with him regularly...I also took my dog to his facility once a week for mentoring. I conquered that battle, but I am not afraid to admit that the next dog will also challenge me and I'll probably need help.
I think the key is that when you ask for help you may not like what you're going to get told, but more often than not, what you don't want to hear is exactly what you need to hear.
OP, I wish you the best of luck. I hope your challenges are met, and you and your horse become a great team, but please don't ask for help and suggestions, then get upset when you don't hear what you want to hear.