Several things:
The regulations I am familiar with include the requirement that any formation flying be briefed and agreed in advance. So, if another aircraft forms up on you without your agreement, that could be a breach of air regulations.
As a responsible pilot, you have to maintain a lookout for less responsible pilots, or pilots who otherwise are not able to exercise appropriate airmanship. There is little excuse for poor airmanship (and no reason I can think of for careless formation flying), but in the case where an aircraft might have to maneuver unexpectedly, it is wise for other pilots to be able to increase their separation if that seems necessary for safety.
Lessons, like day to day flying, don't always go as expected, nor provide the full value of the intended flight. You gotta keep calm and accept that every now and then, it's not going to work the way you planned, and just relax. The notion that a flight has to be accomplished as planned causes "get home itis", which can be fatal - learn to accept and adapt.
A number of times in my flying career, I have been in communication, in busy airspace, and been advised by ATC of an aircraft in my immediate area. Perhaps my vision is too good, but several times, I have seen an aircraft fitting the description, and thought I had visual contact - only to be alarmed by a similar, closer aircraft seen subsequently. Recently I was jump seat in a large transport aircraft, and we were advised of a gyrocopter at our 11 o'clock. The left seat pilot reported the gyro in sight. A moment later, I saw another much closer, which had blended better into the background, and instructed an immediate right turn to prevent a mid air. Never relax about thinking you're aware of all the traffic!
Then, relax and enjoy the flying, planned, or slightly otherwise!