I doubt if this is a strictly translated quotation from Schopenhauer rather than either a very free translation or a summary of his pessimistic viewpoint. In The World as Will and Representation, I, tr. E.F.J. Payne, §16 we find :
life ... is short, fleeting, and uncertain ... life is so full of troubles and vexations, that one must either rise above it by means of corrected thoughts, or leave it.
The phrase, 'short, fleeting, and uncertain', is so very close to your, 'short, questionable and evanescent' that one may be a version of the other. The rest of your quote in not so readily dealt with : 'one must either rise above it by means of corrected thoughts, or leave it' is hardly a plausible translation of 'it is not worth the trouble of any major effort' though the implications for the value of life are pretty much the same.
I have not been able to find anything corresponding even approximately to your quote in World as Will & Representations, vols I & 2, Studies in Pessimism or Parerga and Paralipomena. The quote may derive from elsewhere but these are the most obvious places.
Best I can do, I'm afraid.