Pilot on training flight with Instructor reported NMAC with another aircraft while entering airport traffic pattern for landing.

Narrative

I'm a student pilot with PPL and IR training for a commercial pilot certificate. Was flying a practice medical diversion into ZZZ with my instructor in preparation for an upcoming FAA checkride. Was on CTAF on COMM 1 and monitoring ZZZ Departure in the COMM 2; and I was making radio calls as normal to join the 45 degree entry to the left downwind to Runway XX. Instructor was scanning; and while I was paying as much attention outside as possible; I was preoccupied with the diversion and focusing on finding the airport; making calls; scanning the traffic pattern for other aircraft; etc. My instructor suddenly exclaimed; and I looked up to see another aircraft (a GA aircraft that might have been a Cessna of some sort) heading toward us straight on and probably 200-300 feet above us. I instinctively banked right to avoid them since they were coming from the left (my side of the aircraft in the left seat); but they had passed over us within 2-3 seconds. I continued the practice diversion to a landing and tried transmitting on guard after departing again but could not get ahold of any ATC.