PA28 PVT PLT ATTEMPTS TO EXIT RWY ONTO TAXIWAY AND ENDS UP IN GRASS INSTEAD.

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I HAD PERFORMED 6 LNDGS BEFORE MY FINAL 7TH LNDG. I GOT THE CLRNC FROM TWR TO LAND ON RWY 17R. I FLEW MY PATTERN AND FINAL AS NORMAL WITH NO PROBS. AFTER LNDG; I CAME TO A COMPLETE STOP AND THEN PROCEEDED TO TAXI TO THE NEAREST EXIT TXWY. I PASSED A SIGN THAT INDICATED ALPHA 2 TXWY. I FOLLOWED THE BLUE TXWY LIGHTS UNTIL I SAW WHAT I THOUGHT WAS A BREAK IN THE TXWY LIGHTS INDICATING TO ME THAT THE TURN FOR THE TXWY WAS THERE. AFTER TURNING; I SAW GRASS IN FRONT OF ME. I PANNED AROUND TO LOOK FOR THE NEAREST PAVEMENT; BUT I COULD NOT SEE ANYTHING. BY THEN I STARTED TO GO DOWN IN THE DITCH. I CHOSE NOT TO FIGHT IT OR FORCE THE PLANE IN A DIRECTION. GRAVITY TOOK OVER AND I ROLLED DOWN INTO A LITTLE DRAINAGE DITCH. AFTER THERE; I TRIED TO BRIEFLY MOVE THE ACFT UNDER 'SOFT FIELD' CONDITIONS. IT DIDN'T BUDGE SO I ASKED GND FOR HELP AND THEY DISPATCHED A TOW TRUCK TO GET MY PLANE OUT. BEFORE SIGNING OFF; I GOT CLRNC FROM GND TO SHUT DOWN MY AIRPLANE. I THEN NOTIFIED MY DISPATCH AND LET THEM KNOW OF MY SITUATION. AFTERWARDS; I SHUT DOWN MY ACFT AND PROCEEDED TO POSTFLT IT. NEITHER ME NOR THE PLANE (AS FAR AS I KNEW AFTER POSTFLT AND TOWING) SUSTAINED ANY DAMAGE. THE PLANE ONLY HAD SOME MUDDY TIRES. AS FAR AS WHAT CAUSED THE ACCIDENT; I FEEL STRONGLY THAT IT IS MY INEXPERIENCE AS A PLT. I ONLY HAVE 70 SOME HRS AND ONLY A HANDFUL OF THEM HAD BEEN AT NIGHT. THIS WAS MY FIRST SOLO AT NIGHT WHICH ALSO CONTRIBUTED TO THE SITUATION. AS FAR AS STEPS TO PREVENT IT IN THE FUTURE: IF I WERE TO HAVE SLOWED DOWN (MY THOUGHT PROCESS NOT THE PLANE) AND PANNED THE LIGHTS SLOWLY; I WOULD HAVE SEEN THAT THERE WAS NO TXWY AND PROBABLY COULD HAVE AVOIDED GOING OFF THE SIDE.