C150 pilot reported engine roughness due to carburetor icing resulting in a diversion to the nearest suitable airport.

Narrative

I was conducting a VFR cross-country flight from ZZZ1 to ZZZ2 and receiving Flight Following from ZZZ Center upon departure. While cruising eastbound at 9;500 feet MSL; and approximately one hour into the flight; the engine RPM began to fluctuate significantly and the throttle became unresponsive; resembling an over-lean condition. Suspecting an impending mechanical failure; I immediately initiated troubleshooting by checking the left and right magnetos individually; but the roughness persisted on both settings. I squawked XXXX and advised ZZZ Center of the engine roughness; stating my intention to divert to ZZZ; a private gravel strip located approximately 6 miles away. During the descent through 8;000 feet MSL; I applied full carburetor heat. I continued the descent to the field and circled to lose altitude; during which time the engine responsiveness began to return. By the time I turned downwind; I had regained full throttle control and the engine was running smoothly; but I elected to continue the precautionary landing to inspect the aircraft on the ground rather than troubleshoot in the air. The landing was performed without incident and a post-flight inspection revealed no oil leaks or visible mechanical defects.The root cause of the event was the formation of carburetor icing which I failed to diagnose immediately due to an incorrect mental model. Upon noting the roughness; my immediate cognitive response was to assume a mechanical failure such as a stuck valve or magneto issue rather than an environmental cause. This bias led to a procedural error where I deviated from the standard rough engine checklist by checking magnetos before applying carburetor heat; delaying the resolution of the icing. Despite the engine recovering in the pattern; the aeronautical decision making to land and verify the aircraft's status on the ground prevented the risk of re-entering cruise flight with an unconfirmed diagnosis.