If you fixate on something, you become overly focused on it or attached to it. Don't fixate on your grades — try to enjoy the process of learning!
While the original 19th century meaning of fixate was "make stable," it soon came to mean "gaze upon." It was Sigmund Freud who shifted the definition to something more obsessive. His theories of psychoanalysis included the concept of fixation, an unhealthy attachment to something or someone — and fixate followed. You can fixate on small things (like a stain on your sweater) or big things (like world poverty).