We all do it and when we come back, my output is noticeably better.
Another thing I've found helpful is to define a few activities I should do in my break to avoid getting sucked in by the urgency of work. These include drinking water, chatting with people and letting my mind roam without judgment for a few minutes.
Funnily enough, the biggest problem with taking breaks is the guilt of feeling that time is being wasted. What I've found however is that I've had a large amount of very useful discussions during breaks. Same with generating ideas about how to solve new problems.
Have a specific time when you stop for breaks. I think it was Graham Greene who would stop writing after he hit 500 words, even if he was in the middle of a sentence. Try setting times for those breaks – say, 10:00 am for a morning break, 1:00 pm for lunch, etc.
Just remember that it will take time to adapt to this and to form the habit of taking breaks. Do it a bit at a time and you'll get there.