The devil will try to upset you by accusing you of being unworthy of the blessings that you have received. Simply remain cheerful and do your best to ignore the devil’s nagging. If need be even laugh at the absurdity of the situation. Satan, the epitome of sin itself, accuses you of unworthiness! When the devil reminds you of your past, remind him of his future!
The quotation as it stands comes almost directly from Saintly Solutions to Life’s Common Problems, p. 182, which you can see here at Google Books. Just in case you can’t see it (G/B can be finicky about which pages it will show you), here’s what it says:
St. Teresa of Avila reminds you that the devil will try to upset you by suggesting a thousand false fears or by accusing you of being unworthy of the blessings that you’ve received. He wants to distract you and even trick into ignoring or discarding the graces that God has given you. St. Teresa advises you simply to remain cheerful and do your best to ignore the devil’s nagging. If need be, even laugh at the absurdity of the situation: Satan, the epitome of sin itself, accuses you of unworthiness. Furthermore, as the saying goes, “When the devil reminds you of your past, remind him of his future!”
This is very good advice, but you’ll note that it’s paraphrasing St. Teresa, not quoting her directly. The direct quotation in the final sentence is not from St. Teresa herself–it’s “as the saying goes,” implying that it’s a common saying, which a Google search will quickly show is indeed the case. It’s attributed to all sorts of people, which is never a good sign for a real quotation.
The next time I spotted it, it showed up in EWTN’s daily devotional for August 31, 2005, which you can view here. The source is obviously the paragraph I just quoted above, modified slightly and attributed directly to St. Teresa. From there it seems to have conquered the Catholic world. I’ve used the saying myself many times in confession and probably a time or two in a homily. I guess I’ll have to quit attributing it to St. Teresa.