Excerpted from the Tanya, Chapter 27, by the first Admor of Chabad, the Baal HaTanya, Rabbi Schneur Zalman of Liadi.
"However, should the sadness (in serving G-d) stem not from worry over sins, but from evil thoughts and desires that enter his mind – if they appear not during Divine Service but while he is occupied with his own affairs and with everyday matters, he should, on the contrary, be happy in his portion that, though they enter his mind, he averts his mind from them in order to fulfil the obligation, That you seek not after your own heart and your eyes which lead you astray (Bamidbar, 15:39).
"This verse does not speak about Tzaddikim, to refer to them as going astray, G-d forbid, but of people of intermediate standing (benonim) like him, in whose mind do enter sexual fantasies, whether of an innocent nature or otherwise. When he averts his mind from them, he is fulfilling the commandment of the verse. Indeed, the Rabbis of blessed memory have said, 'He who has passively abstained from committing a sin, receives a reward as though he had performed a precept' (Kiddushin 39b). Consequently, he should rejoice at his compliance with the injunction as when performing an actual positive precept.
...For this is the nature of the service of the intermediate level (benoni) – to subdue the evil impulse and thought rising from the heart to the brain, and to completely avert the mind therefrom, thrusting the temptation away with both hands, as has been explained (Tanya, Ch. 12). And with every thrust that he expels the fantasy from his mind, the sitra achra down below is suppressed, and since "the stimulus from below causes a stimulus from above" (Zohar 135b), the sitra achra above which soars like an eagle, is also suppressed, in accordance with what is written, 'Though thou exalt thyself as the eagle, thence I will bring thee down, says the L-rd' (Ovadia, 1:4). Thus the Zohar, Parshat Trumah, extols the great satisfaction before the Holy One Blessed Be He when the sitra achra is subdued here below, for then the glory of the Holy One Blessed Be He rises above all, more than any other praise, and this ascent is greater than all else (Zohar 128b).
"Therefore, no person should feel depressed, nor should his heart become exceedingly troubled, even though he be engaged all his days in this conflict, for perhaps because of this he was created, and this is his service – constantly to subjugate the sitra achra."