You are paying the lease so you can spend your time any way you see fit -- period. But, please do not tell me that this is part of 'Natural Horsemanship'. This horse is not fooled into thinking you are a horse and there is nothing 'Natural' about anything we do with horses.
A horse that 'does not allow just anyone on his back' has much more than 'trust issues'; He is just plain poorly trained. There are many ways to overcome improper or non-existent training. It does not take weeks and months -- just good, sound training techniques.
Buzz words, like Natural Horsemanship don't mean anything. It is like paying more for produce that says it was grown naturally with pure spring water. You can test it for everything known to man and all you are getting for your money is 'less produce for the same dollar'.
When you take weeks and months to accomplish what a good trainer can get done in a few days with no abuse and no unreasonable amount of pressure, all you have done is taken more time; you don't get a better horse, a safer horse or a more trusting horse; you only took a lot more time to get it there -- if indeed, it is as well trained -- which most are not.
I find that many people that are so proud of their 'naturally trained' wonder horses, really have very poorly trained horses. If they were to hand the lead-rope to a stranger and tell them that they could take the horse and go ride it, it would probably come with a long list of what the handler/rider had to do to get along with the horse. This means that most of that extra time spent did not produce a better trained horse but only produced a better trained rider that knows what the horse allowed and did not cross certain boundaries that would have upset the horse.
If you find that many professionals have a negative opinion of 'Natural Horsemanship' it is because we have seen very few people bragging about how they used 'natural Horsemanship' to train their horse and they actually have a well-trained, well-mannered obedient and useful horse. We usually see either sour, mad horses that do as little as possible for their riders or we see poorly trained horses that have trained their riders instead of the other way around. Good trainers just go along and get the job done and have obedient, well-mannered, well-trained horses that anyone can get along with and ride.