I can't concentrate at work, please advise

My job includes reading a lot of documents and searching for information in the internet.
I have my own desk and a computer which I use most of the time, and I "work" quietly,
with limited time to talk with my workmates, like the other people in my office.

I would appear to be seriously working or reading the documents but my mind wanders on other things, such as sex, family, future plans, etc.

Thinking about these things reduces my productivity and I'm not happy about it,
but my mind wanders on things other than work. I couldn't concentrate when those images start popping in my head. Sometimes I had to work until late to finish a project,
to compensate for the time wasted on daydreaming and thinking about other things.

Has anyone have a similar experience?
Please advice on how can I concentrate in my job and maximize my productivity.