Likutei Moharan 72:5/ליקוטי מוהר"ן ע״ב:ה׳

15. Depression, too, is very harmful and strengthens the coarse and vulgar evil inclination. Thus, on the contrary, if a person wants to be God-fearing and to pray, and does not want these bad thoughts, he should ignore them entirely. Their presence should not matter to him. Rather, he should attend to what he is involved in—be it Torah or prayer or business—and pay no attention at all to [these thoughts].

2This is like the story I heard about a certain person who, when he got up to pray, would imagine a non-Jewish idolater appearing and standing in front of him. He suffered terribly from this. The more he tried to overcome and remove this thought, the more it gained the upper hand. A sage advised him to not be concerned about it; to let the idolater stand there, and even so to go about his business and pray. By doing so, it would leave him. This, however, is only a temporary solution; for as long as he still has not sanctified his body. For the main thing is to sanctify and purify himself. And he should go to a sage, who will provide him with proper counsel for every matter, as explained above.