MDT CREW HAD A SERIES OF PROBS ON ARR AT DFW.

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WHILE BEING VECTORED FOR APCH TO RWY 17L FROM THE NE; A SPD REDUCTION TO 180 KIAS AND THEN A DSCNT FROM 5000 FT TO 3000 FT WAS ASSIGNED. I (PF) WAS SLOWING TO 180 KIAS AND THEN BEGAN MY DSCNT AS ASSIGNED. AT APPROX 3500 FT WE WERE REASSIGNED RWY 17C BY APCH. PNF SAID THAT WE HAD IT IN SIGHT AND I BEGAN MY L TURN TO FINAL. WE SWITCHED TO TWR AND WERE CLRED TO LAND. AS I WAS TURNING ON FINAL TO WHAT I THOUGHT WAS RWY 17C; ANOTHER ACFT APPEARED ON TCASII AT 200 FT BELOW AND 2 O'CLOCK (RANGE APPROX 3/4 - 1 1/4 MI). TA ISSUED BY TCASII BUT NO RA. WE ACQUIRED THE TFC VISUALLY. WHILE TURNING BACK TO THE L TO AVOID TFC; TWR INFORMED US WE WERE LINING UP FOR THE WRONG RWY. TWR AND US REALIZED THIS AT ABOUT THE SAME TIME. I TURNED FURTHER L TO REALIGN THE ACFT WITH RWY 17C. WE LANDED WITHOUT INCIDENT BUT I WAS EXTREMELY ANGRY WITH MY LOSS OF SITUATIONAL AWARENESS. A LATE CHANGE OF RWY FOLLOWED BY A LATER THAN NORMAL HDOF TO TWR HAD ME THINKING WE WERE IN A DIFFERENT POS THAN WE REALLY WERE. (I THOUGHT WE WERE FARTHER FROM THE ARPT). YET ANOTHER EXAMPLE WHERE A FEW MOMENTS OF INATTN/LACK OF SITUATIONAL AWARENESS COULD HAVE CAUSED AN INCIDENT. MY GUARD WAS LET DOWN AND I LEARNED THAT CONSTANT VIGILANCE IS THE SOLUTION.