The problem is that the workplace is filled with constant interruptions. Either a coworker needs to talk to you about something, you need to get up to go talk

The problem is that the workplace is filled with constant interruptions. Either a coworker needs to talk to you about something, you need to get up to go talk to someone, or you end up interrupting yourself when you should focus on working by checking email, browsing to a website, or any million other things. To improve your attention span, you need to work at removing as many of these restrictions as you can. Since you can't control the external distractions - coworkers, meetings, etc., that means you have to focus on the ones under your control. So for start, pay attention to what you're doing at any given time as you work and try to reduce the number of times you interrupt yourself.