RE: when to stop trying
First... Have a good trainer aircraft... not a $70 cheapee.... Sig Senior or Telemaster or LT 40.... Second... a nice engine... O.s. 46 FX up to 70 Four stroke... Power helps the instructor recover an aircraft going down... Third... At our age you need an instructor that helps you fly and prevents crashes.... an instructor that says I wanted to see if you could save it needs to be using his airplane to teach... (There are a bunch of guys that will let you crash)....
Fourth or should this be FIRST..... Have fun, be relaxed and fly.... Fifth and last... concentrate on your flying... do not look around... do not talk except to the instructor and do not let anyone push you past your abilities .... unless again they want you to fly their airplane...
several good comments have been suggested... have fun and only look at your airplane and discourage those that want to see you crash... there is always one at the flying field... break a few props... fix the airplane and fly fly fly... you will be glad you did... there are so many great guys out there that want to help you... look for them .... you will be glad you did....