There is so much more to life than being productive. Many of us are living with "internalized capitalism".
Something that helps me get over that feeling of not-doing-enough is time tracking my work (using a tool like Toggl). After a few months you get the sense that a productive day for you is X hours of work, and that makes it easier to disengage from work once you hit that point in your day. Knowing what your "X" is per day also justifies taking a break at hour X+2 because you know that you're straining your body at that point.
Something that helps me get over that feeling of not-doing-enough is time tracking my work (using a tool like Toggl). After a few months you get the sense that a productive day for you is X hours of work, and that makes it easier to disengage from work once you hit that point in your day. Knowing what your "X" is per day also justifies taking a break at hour X+2 because you know that you're straining your body at that point.