You will never catch me helping new fishkeepers again. There is no point in wasting my time if they are not going to listen.
One of the basic truths of working in sales is that you must have a thick skin and be able to tolerate rejection; it's often stated that you will hear a new prospect say "no" at least 7 times before you get a "yes". And when you are trying to help a new aquarist, you are essentially selling an idea, so...you gotta get used to the feeling that you are talking to a wall.
I spent about a decade in commissioned sales before getting completely fed up and moving on to an honest job. You just get to the point where your patience is gone, and the self-control that has kept you from launching yourself across a desk and grabbing some idiot's throat is wearing dangerously thin. I vividly remember that specific customer that was my "last straw"; I had been feeling the signs of burnout for awhile, and the customer in question displayed a heady mix of stupidity, arrogance, stubbornness and distrust, with just a dash of self-righteousness thrown in for spice. When I caught myself starting a sentence with "Look, lady..." I just decided then and there that it was time to move on.
The problem you're facing is much worse, because I was doing it to earn a living, i.e. for my own benefit...but you're getting worn out by something that you are doing essentially selflessly, in a simple effort to assist somebody else with no personal gain for you. There's nothing wrong with just saying "the hell with it!" and withdrawing partially or completely from the arena for awhile; it might be the best thing for you. You have no obligation to stress yourself out for no reason.
After a period of simply watching a succession of these jokers go down in flames of their own creation, you'll probably forget yourself and, without thinking, respond to one of them. Who knows? Maybe it will go well for both of you...but the odds are that it will be another one of
those...and the encounter will be a cold slap that knocks you back out of the ring for awhile.
I'm trying to avoid engaging these types as well; periodically making a foray into a conversation with one of them, and so far at least I am almost always regretting it. A neighbour that had some goldfish...and then decided that she didn't want them and gave them to me...and then decided a short time later that she wanted to try goldfish again, so she bought another batch...recently invited my wife and me over for coffee. As we sat on the deck, she casually asked me if I wanted to see her fishtank. I almost said "not really" but my wife piped up "oh, yes, let's see it!" so we trooped into the house, to be faced with her fishtank and it's new goldfish...and a pair of small angelfish...and a pleco...and a tiny Oscar.
I could feel my wife's gaze on me, as she wondered what I was about to say...so I just said "that's nice".
Take a break, and it'll get better...
...won't it?
