What is the difference between disability and difficulty?

The difference between difficulty and disability is hard to determine. In a broad sense, a difficulty causes a task to take longer or require more effort to complete, but it remains possible to perform the task. A disability prevents the performance of the task.
Examining specific cases is easier to analyze the difference between disabilities and difficulties. A learning difficulty is classified and includes diagnoses such as Attention Deficit Disorder (ADD) and dyslexia. Sufferers can still learn, but often require coping mechanisms. People with learning disabilities have a mental impairment which affects their learning. 
Outside the world of education, the distinction becomes more difficult to discern and much more emotional. The difference is more technical and any label should be carefully reviewed. A person with a missing leg is technically disabled because that person cannot walk without an artificial aid. Still, that person might not call themselves disabled because he or she can achieve the task with mechanical coping mechanisms. 
The inability to complete a task does not imply a disability. The inability must be caused by some defect which prohibits completion under any circumstance. An overweight individual may not be able to complete a marathon, but that does not create a disability for him or her because it is still theoretically possible to train to complete the task.
https://www.mentalhealth.org.uk/learning-disabilities/a-to-zl/204300