[When one is plagued by negative thoughts and he subsequently banishes them from his mind...] one should not feel depressed or very troubled at heart. He ought to be somewhat troubled by the occurrence of these thoughts, otherwise he may become indifferent to them and will cease to wage war against them. But he ought not to be sorely troubled by them, even if he were to be engaged constantly in this conflict with the thoughts that will always enter his mind. Though he may never rise to the level that precludes their occurrence, he should not be depressed. For perhaps this is what he was created for and this is the service demanded of him - to subdue the sitra achra (the side of unholiness) constantly.

Tanya, ch. 27