Because I’ve been bumping into this one all over the place lately.
Things that are discreet are quiet, unobtrusive, not-noticed, and don’t draw attention to themselves. “Smith made discreet inquiries into Jones’s financial history.”
Things that are discrete are separate and distinct from each other. “Jones set up discrete budgetary categories for his various expenditures.”
Got that?
Good.
Dr. Doyle's Blog 
And I was going to say I need a good mnemonic to remember which is which, when I realized that the Es are separated by the T… discrete=separate
Got it 😉