I recently made a switch from an advertising agency (where I was working on many different website for as many different clients) to a product company where I just work on a single aspect of a single product that has been in development for many years.

We use scrum and I find that the environment and this particular way of programming in a team has made an incredible difference in my ability to focus on a single task. What I mean to say by this, is that sometimes (and for some people) it may help to you change your surroundings if you have trouble with focus.