Strikes me you have three choices:

1. Force yourself to sit here and do nothing else until the task is done. The Chandler method, which requires a great deal of discipline.

2. Keep telling yourself "I'll start just after I check Hacker News / Twitter / watch one more episode on Casualty". This means you never achieve anything and feel forever guilty and worthless.

3. Put the task to one side for now and do something else enjoyable and marginally useful. You make progress in other tasks, feel valuable, and only a little guilty.

Structured Procrastination is the third option. If you can't manage Option 1 -- and who can? -- it's better than Option 2.