It's getting harder to focus every day

I’m feeling it right now. I had to set a timer for 15 minute on my computer and block all distractions to write this. If I didn’t force myself to focus I would easily get distracted by something after few minutes. Even when I’m doing things that I’ve been waiting to do it, I still feel the urge to do something else.
I don’t know how and when this happened. During the last few years I was always studying, working, and doing open source. And actually got stuff done. Doing all of those at the same time requires paying attention to what I wanted to do and ignore the noise.