A low time small aircraft pilot entered the SAC Runway 20 traffic pattern incorrectly because his mental model of how the traffic pattern was constructed did not fit ATC's instructions.

Narrative

On a flight to SAC; I approached the airport from the south west; passing 1 NM west of the SAC VOR. On initial call up; I recall Tower instructing me to enter right traffic for Runway 20 and report downwind. I maneuvered to enter the standard left pattern for Runway 20. While entering downwind; I noticed a departing aircraft making a left crosswind turn. I notified Tower and requested a right 360 as it appeared that this would keep me clear of the aircraft's path on crosswind. Tower asked who was requesting a right 360. Once Tower understood who and where I was; Tower asked if the departing aircraft had me in sight (they did) and advised that I continue on downwind in the left pattern. The departing aircraft passed behind me below my altitude. A contributing factor to this event was the lack of terminal radar services; this was announced on ATIS. It was also a busy time; with multiple aircraft arriving and departing; including two aircraft in the pattern. While briefing myself for the approach after picking up ATIS; I reviewed the standard pattern for the Runway in use; 20. The standard pattern is left traffic. On a flight with my instructor to this airport; I had made the mistake of thinking I was to enter right traffic; when in fact at this earlier time I was instructed by Tower to enter left traffic. In reviewing the pattern information; I created a sense of certainty about what to expect; to the degree that I believe I read back Tower's instruction to enter right traffic without realizing the conflict with what I was expecting and changing my mental plan for the approach to match Tower's instruction. Since I thought Tower's instructions matched what I expected; I didn't write them down. Due to the lack of radar I placed a greater emphasis on looking out for other traffic; perhaps to the extent that I mentally moved on too quickly after Tower's pattern instruction. While reviewing standard approach information and being prepared for what is likely to come is always a good thing to do; I will go one step further in the future to include alternatives; such as 'what if I'm not assigned standard traffic; what will I do?'. Given my position on initial call up; perhaps I should have anticipated that Tower would give me right traffic to decrease the distance traveled for pattern entry. I will try to write down ATC instructions; even when they match what I expect and are something I think I can remember. Had I written down the instruction; I might have caught my error by doing so.