The long term studies are not that long. There are studies following kids into adulthood ~6 years to past college. These studies have reported a less likelyhood of substance abuse when ADHD meds are used than with untreated ADHD. This is thought to be due to self-medication and poor coping skills due to dealing with the difficulties of dealing daily with ADHD for so long (school and job failure, impulsivity, etc.). Substance abuse and addiction is also more likely when ODD is present.
Methylphenidate has been used routinely for ADHD for over 50 years and if there were issues, they would be showing up. Adderall used for ADHD is in a much smaller dose (than abuse) and given through a route (orally) that is least likely to lead to addiction. If you are concerned about your specific issues, please discuss these with a counselor (do you have a psycologist for counseling or possibly an ADHD coach?). Having someone to discuss this stuff with can be very valuble.