bite off more than one can chew To undertake more than one can handle; to overextend one-self. John H. Beadle used the phrase in Western Wilds, and the Men Who Redeem Them (1877).

burn the candle at both ends To over-extend one-self; to overdo; to use up or squander in two directions simultaneously. The phrase often carries connotations of dissipation. It comes from the French expression brusler la chandelle par les deux bouts, and first appeared in Randle Cotgrave’s A Dictionary of the French and English Tongues (1611).

lazy man’s load A burden too heavy to be carried; a task too large to be completed. This expression alludes to the purported tendency of lazy people to overburden themselves on one trip rather than make two trips with loads of a reasonable size.

serve two masters To split one’s energies between pursuits of good and evil, uprightness and decadence, kindness and cruelty, etc.; to attempt to adequately meet conflicting demands; to work against one-self. This expression, of Biblical origin, alludes to the self-defeating nature of the impractical if not futile attempt to obey two opposing sets of ideologies, morals, or ethics.

spread one-self thin To overextend one-self, to be involved in so many projects simultaneously that none receives adequate attention; to overdo, to have too many irons in the fire. This popular expression compares a person’s limited capabilities and resources to a given amount of a literally spreadable substance, such as jam or butter, which can cover just so much bread before it becomes too thin to be tasted.

too many irons in the fire Too many projects requiring one’s attention, to the detriment of them all; so many undertakings in progress that none gets adequate attention. This expression, in use as early as 1549, refers to the pieces of iron a blacksmith heats in the forge before working on them; they must be hammered into the desired shape at precisely the right temperature. If he tries to prepare several at once, his efforts become counterproductive: he either gives short shrift to working the metal, or risks overheating it so that its malleability is adversely affected. A similar phrase, many irons in the fire, has the more positive meaning of several alternative ways to achieve one’s ends.