A general aviation pilot reported they mistakenly followed aircraft on approach to a parallel runway instead of traffic on approach to their runway resulting in a NMAC.

Narrative

Was given directions to make right traffic to RW XXR for #2 behind a C150 on 2 mile final. There was also traffic utilizing the parallel runway; XXL. I advised I had the traffic in sight and was cleared to land #2 behind the landing traffic. I advised turning base as I observed the aircraft abeam my position and was again given landing clearance #2 behind the Cessna. Rolling out on base; I lost sight of the aircraft; when it would have been on short final. I continued on final and began descent (fairly steep; I stay at pattern altitude until a power-off descent to the runway).I increased my rate of descent with a slip and then noticed an airplane below me crossing the numbers on XXR; estimated to be 150ft below. I declared a go-around and side-stepped to the right and was given direction to make right traffic and cleared to land #1 for XXR. No further discussion with ATC! I believe that the traffic I 'confirmed in sight' must have been traffic on the parallel runway and I never saw the traffic I was to follow. Training aircraft at ZZZ are often at very low altitude on extended finals and are hard to see in the ground clutter. I also received no traffic warning for this aircraft but did have multiple other aircraft in sight and displayed via ADSB in the vicinity; but not the one in conflict.