PLT OF PA28 SUFFERS SPATIAL DISORIENTATION AND LOSS OF SITUATIONAL AWARENESS AFTER A LONG OVER WATER FLT TO OPF.

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AFTER A LONG OVERWATER FLT; I BECAME DISORIENTED ON APCH FROM THE E TO THE OPA LOCKA ARPT (OPF). FOR SOME REASON I HAD BEEN EXPECTING TO LAND TO THE W; BUT THE WINDS THIS DAY WERE FROM THE E. THE CTLR INSTRUCTED ME 10 MI OUT TO ENTER THE L DOWNWIND FOR RWY 9L; BUT I BECAME CONFUSED; PROBABLY DUE TO THE EXHAUSTION OF THE LONG FLT. I BELIEVED I WAS LINED UP FOR FINAL ON RWY 9L WHEN I WAS LINED UP FOR RWY 27L. THE CTLR CONTINUED TO INSTRUCT ME TO GO TO THE DOWNWIND LEG; BUT THIS WASN'T MAKING SENSE TO ME. I WAS FIXATED ON THE RWY I WAS APCHING. ABOUT 4 MI OUT; I REALIZED MY MISTAKE; BUT BY THAT TIME I WAS FAR OUT OF THE POS WHERE I SHOULD HAVE BEEN. LUCKILY; THERE WERE NO OPS ON THE S SIDE OF THE ARPT; OTHERWISE I COULD HAVE CAUSED A CONFLICT. I DON'T FULLY UNDERSTAND WHY I BECAME FIXATED ON BELIEVING THAT I WAS ON APCH TO RWY 9L WHEN IT WAS ACTUALLY RWY 27L. FACTORS MAY HAVE BEEN MY FLYING VISUALLY FROM THE R SEAT (SO AS TO BE NEAREST THE DOOR IN CASE OF EMER DITCHING); SUCH THAT THE DIRECTIONAL GYRO WAS NOT AS MUCH IN MY SCAN AS IT SHOULD HAVE BEEN. I ALSO HAD NOTICED DURING THE FLT THAT THE OXYGEN FLOW TO MY MASK WAS SLOW AT ALT DUE TO A LOOSE FITTING. I TIGHTENED THE FITTING; BUT IT IS POSSIBLE THAT I HAD BEEN SUFFERING FROM LACK OF OXYGEN DURING THE FLT. IT IS HARD FOR ME TO SEE HOW THIS WOULD HAVE AFFECTED ME AT 1000 FT MSL; BUT IT MIGHT HAVE ADDED TO THE EXHAUSTION FACTOR.