How can an adult overcome a learning disability he's had for so long?

Let's talk. The type, severity, & pervasiveness of the ld will determine its treatment. Obviously, the ld needs to be described & appropriate interventions made. Yes, it's possible to overcome a long standing ld if the patient has the desire and fortitude to try. Many appropriate accommodations occur in ld patients, some of which they are entirely unaware of. Seek help from developmental doc & therapists.
Evaluation. The first step in dealing with a learning disability is understanding the cause and how it is affecting you. This requires a thorough neuropsychological evaluation which should lead to understanding your problem and provide a basis on which suggestions for strategies and approaches can be developed. Ask your doctor for a referral to a Neuropsychologist.