Perseverance or insanity?

I do not think that there is any other quality so essential to success of any kind as the quality of perseverance. It overcomes almost everything, even nature. - John D. Rockefeller

We've probably all heard the great quotes on the importance of perseverance, right?  There are many, including the one above and the one about getting knocked down 6 times and getting up 5--or is it the other way around?  There are, in any event, all sorts of good things that are said about the value of just keeping on keeping on. 

There's one problem, though.  Another relatively famous quote that's been attributed, near as I can tell, to almost every smart person in the history of the world, tells us "the definition of insanity is doing the same thing over and over and expecting a different result." 

At what point do perseverance and insanity collide? When do you stop doing what you've been doing the same way you've been doing it? 

Last week I queried eleven agents.  I've received five rejections.  Not too bad, really, from what I hear.  Part of me, I confess, wants to give up and just start spending the evenings with my family instead of slaving away over a laptop keyboard.  It's a small part, mostly due to the fact that I've heard all about the value of perseverance.  But--here's the big but--do I keep querying the same way, or do I rewrite my query letter?  At what point do I say "this isn't getting the job done" and scrap it for a revised version? 

I've got five more agents on the list to query, and I'll have a query out to each of them tonight.  I think I am going to change the way my query letter is written somewhat, try a little different approach, and see if it works better.  I only need one yes, after all. 

Well, here goes insanity.  Or is it perseverance?