if you dont mind me asking... what kind of shape are you in? If I understand correctly, you have been riding for years, you own 4 bikes and had three others, and after years of practice you are at 12 miles max, and when you free-form it you do half that.
how often do you ride?
I'm beginning to think you either need to go out a lot more frequently to build up your endurance, or its a complete mental issue, in which case a previous posters idea of having somebody drive you to 15 miles away from your house and then you HAVE to ride more then 12 to get home.
If you want, come on up to nashua and I'll ride the rail-trail with you. Nice and relaxed and a great view.