C172 PILOT REPORTS NOT ACQUIRING TRAFFIC PRIOR TO TURNING BASE RESULTING IN INSUFFICIENT SPACING AND GAR.

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I WAS PRACTICING TKOFS AND LNDGS ON RWY 28R IN A R PATTERN. AFTER 4 TKOFS AND LNDGS THE TFC PATTERN WAS BECOMING INCREASINGLY CONGESTED WITH APCHS AND DEPS OF BOTH PLANES AND HELIS. ON MY 6TH TKOF; IN AN APPARENT ATTEMPT TO ALLEVIATE SOME OF THE CONGESTION; THE CTLR INFORMED ME THAT HE'LL CALL MY XWIND. AS I CONTINUED ON AN EXTENDED UPWIND I HEARD INBOUND TFC FOR BOTH RWYS 28L AND 28R. THE CTLR THEN CALLED MY XWIND. I WAS SOON CLRED TO LAND: 'C172 #2 TO FOLLOW A CESSNA ON R BASE; RWY 28R; CLRED TO LAND.' I ACKNOWLEDGED THE CLRNC AND SCANNED FOR TFC; BUT WAS UNABLE TO SEE IT. AT THE 45 DEG ANGLE TO RWY 28R I GLANCED AROUND AND SAW TFC ON RWY 28R OR ON A TURNOFF FROM RWY 28R AND INCORRECTLY ASSUMED THAT THIS WAS THE '#1.' I TURNED ONTO BASE. I SAW A CESSNA TO THE L OF RWY 28L THAT APPEARED TO BE HDG TO RWY 28L. I HAD HEARD LNDG CLRNCS FOR RWY 28L AS WELL AND INCORRECTLY ASSUMED THAT THIS PLANE WAS HEADED FOR RWY 28L. AS I TURNED FINAL IT VERY SOON BECAME APPARENT THAT THIS CESSNA WAS INTENDING TO LAND ON RWY 28R. THE CTLR SOON SAID 'C172 YOU'RE RIGHT BEHIND THE CESSNA...WHAT ARE YOU GOING TO DO...GO AROUND?' TO WHICH I STATED; 'AFFIRMATIVE; WE'LL GO AROUND.' WHILE THERE WAS AMPLE ROOM TO MANEUVER AND I WAS ABLE TO EASILY CLB BACK TO PATTERN ALT THE ENCOUNTER WAS FAR TOO CLOSE FOR COMFORT. I BELIEVE THE SITUATION OCCURRED BECAUSE: I DIDN'T HAVE THE '#1' TFC IN SIGHT IN THE AIR AND INCORRECTLY ASSUMED THAT I SAW IT WHEN I SAW ANOTHER PLANE ON THE GND. I INCORRECTLY ASSUMED THAT THE TFC THAT I DID SEE; TO THE L OF RWY 28R WAS DESTINED FOR RWY 28L. I DIDN'T ANNOUNCE MY FAILURE TO SEE THE TFC INITIALLY. THE OTHER CESSNA EITHER CAME FROM THE SW ON A L BASE FOR RWY 28R OR CAME FROM THE E BUT DRIFTED TOO FAR L BEFORE RETURNING TO RWY 28R. (I THINK THE FORMER IS MORE LIKELY BECAUSE I DIDN'T SEE THE CESSNA WHEN I WAS ON DOWNWIND; VISIBILITY WAS 10 SM; AND THERE WAS A SLIGHT XWIND FROM L-TO-R ACROSS THE RWY; SO A PLANE WOULDN'T LIKELY DRIFT TO THE L.) TO PREVENT A RECURRENCE: I CAN AVOID CONGESTED PATTERNS UNTIL I AM MORE PROFICIENT. I NEED TO TAKE INTO ACCOUNT THAT PLANES MAY NOT BE CORRECTLY APCHING THE RWY ON WHICH THEY INTEND TO LAND AND AVOID ANY ASSUMPTIONS ABOUT THIS. I NEED TO VERIFY CLRNCS AND CLEARLY IDENT PLANES SEQUENCED PRIOR TO ME. I NEED TO INFORM THE TWR IF I AM UNCERTAIN. IT WOULD ALSO BE HELPFUL IF; AS SOON AS I TURNED BASE OR FINAL; THE CTLR WOULD HAVE IMMEDIATELY INFORMED ME THAT I WAS TOO CLOSE RATHER THAN WAIT.