Before you even start looking into leasing you should make sure you have your parents complete ok. After all they do have control over you until you're 18. lol
Off-farm leases can be very difficult to find as you have seen for your self. Mainly because during an off-farm lease a horse can pick up nasty habits or even be ruined, and no good owner wants that. Most responsible owners want what's best for their horse and want to be able to keep an eye on said horse.
Also, the only cases of off-farm leases I've had experience with required the lessee to use the owner's trainer, vet, and farrier so that the horse would be taken care of to the owner's standard. Plus, I don't know of anyone who would let a trainer they don't know or trust do anything with their horse. I know I wouldn't. I love my boys too much to take a chance with a stranger.
I thought you were able to use you trainer's horse Secret, and that was basically like you owned her? A new horse won't be a miracle solution to any issue you have with Secret. In horse riding 98.5%(if not more) of issues are rider error, and are transfered onto any horse you get on. It's up to the rider to know their weak spots and work to improve them. No one is perfect. I've got multiple titles and championships under my belt, and I know I'm still far from perfection. As riders we must take responsibility for ourselves and work to better our riding. When things go wrong on the horse in 99% of cases the rider has no one to blame but her/herself. You don't get better by blaming your shortcomings on others. I think your best bet would be to stick with Secret or maybe another horse of your trainer's. Off-farm leases are hard to come by and not always the best solution.
<3 Dallas, Dakota, and Shaymus <3
RIP Shaymus 8/16/13