RWY INCURSION WHEN A C152 CFI PLT CROSSES RWY 23L THINKING HE HAD PERMISSION TO DO SO AT RDU; NC.

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TWR CLRED ME TO LAND ON RWY 32. UPON LNDG; TWR SAID 'AT FIRST EXIT; CONTACT GND POINT NINER.' I SWITCHED OVER AND GND SAID I SHOULD BE WITH TWR UNTIL CLR OF RWY 23L; SO BACK WITH TWR WHO SEEMED RATHER BUSY; SAID 'HOLD SHORT RWY 23L.' I HELD AS REQUESTED. TWR; 'ACFT Y (A HELI) EXPEDITE XING; RWY 23R; CONTACT GND POINT NINER OTHER SIDE.' I THOUGHT SHE SAID 'ACFT X; CROSS RWY 23L; CONTACT GND POINT NINER.' I SHOULD MENTION AT RDU YOU HAVE ONLY 1 LOCATION ON THE WHOLE ARPT THAT YOU CAN CROSS A RWY AND THAT'S AT MY HOLDING LOCATION (UNLESS YOU'RE A HELI). I CROSSED RWY 23L REALIZING AS I CROSSED THAT I ALSO HAD AN INBOUND ACFT THAT HAD TO GO AROUND BECAUSE OF MY ACTIONS. THE PLT OF THAT ACFT SAID HE WAS LISTENING AND UNDERSTOOD WHAT HAPPENED AND FELT THE GO AROUND WAS UNNECESSARY; BUT WE BOTH UNDERSTAND THE RULES. THE TWR SUPVR AND I TALKED FOR 30-40 MINS AFTER THE INCIDENT. HE WAS GREAT ABOUT THE WHOLE THING. I WAS REALLY UPSET AT WHAT I HAD DONE. HE ADVISED ME THAT IT WAS A TRAINING NIGHT AND IT WAS A BUSY ONE IN THE TWR. HE LISTENED TO THE TAPES AND WE TALKED ABOUT IT A WHILE. HE ALSO SAID IF THE FAA WANTED TO TALK TO HIM ABOUT IT HE WOULDN'T MIND EXPLAINING TO THEM ABOUT EVERYTHING THAT TOOK PLACE. WHAT COULD I HAVE DONE TO PREVENT IT: I COULD HAVE ANGLED MY C152 INTO THE APCH END DIRECTION SO MY WING WOULD NOT BLOCK MY VIEW OF THE INBOUND TFC. I SHOULD HAVE READ BACK MY INSTRUCTIONS AND WAITED FOR CONFIRMATION BEFORE XING. DON'T BECOME COMPLACENT WITH THE REPETITION OF THE FLTS (ALWAYS THE SAME).