How do you focus for long periods of time?

Unfortunately my IT work is being impacted due to the fact that I get distracted very easily and have a hard time focusing. I have been diagnosed with ADHD in the past. I have taken adderall before, but I gain tolerance to it and it doesn't work after that. The problem is the fear I feel of failure really screws with my head and doesn't go away and I have a very difficult time being productive. How can I fix this?

How do you focus for long periods of time?
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