I spoke to a fairly well respected psychologist about exactly the issue OP was describing. He felt that the issue was really poor impulse control and suggested introducing delays between action -> reward.

He thought it wasn't ADHD because I don't otherwise get distracted if I'm engaged (watching a movie in a theater, cooking etc) and can generally see the task through to the end.

Note: watching a movie in a theater is, for me, a totally different experience to watching it at home. At home there's always an option to pick up the phone in the middle or walk away and do something else whereas at the theater I don't have that option. I can very easily watch a full movie at the theater but it's been a fairly long time since I was able to watch a full movie at home in one go.