I think what you are describing is normal, and this common bad habit can be exacerbated by certain factors outside of ones' control.
I try to combat this issue with:
1) Very regular sleep schedule, even on weekends. For me, the difficulty focusing is often a sign of fatigue, even if I do not actually feel tired.
2) If I am managing to focus, I roll with it as long as I can. I do not answer the phone, check that e-mail that just hit my inbox, or do anything else until the wave of productivity and focus has ended on its own accord. If a coworker approaches me in person, I just say "sorry, working on a short deadline, I'll find you later."
I will say, though, that there are days where I am totally engaged and I do not once check the news, my personal e-mail, my favorite blog, etc. If that never happens for you, maybe you could look for a way to get more motivated about your career.