This is probably not the best place to seek expert advice. The responses you'll get here will either be based on personal experience, or folks will post stuff they found on the Internet--the same stuff, in other words, that you could find yourself.

In your place, I'd start by talking to my doctor and insisting on some answers. If I wanted to get my hands on multiple long-term studies I'd go to the nearest college or university with a decent hard sciences program and ask the librarian to help me locate the studies--if they exist. (Amphetamines have been used for over 50 years, so I'm sure you'll find some long-term studies, but I doubt very much you'll see anything long-term that specifically addresses Adderall XR since this is a relatively new drug.)

Here's the little bit I do know:
Adderall is a stimulant, and Adderall XR is a form that's designed to reduce the number of dosages you need to take--usually reduces from twice daily with Adderall to once daily with Adderall XR. Like all stimulants, it increases the amount of certain chemicals in the body, thereby stimulating the central nervous system. This is intended to help you concentrate. (We know that it works, but like most meds, we don't know exactly how and why it does so.)

I hope you find what you're looking for. Don't be afraid to press your doctor for answers.