Musar על ברכות 9:2

And even if some time transpires in which, for some reason, one cannot spur himself to this, or in which he sometimes finds himself defeated in this by the yetzer, still, he must not allow himself to despair of ever attaining it. For if he strengthens himself further and arouses himself to this guarding, he will emerge the victor and will conduct himself accordingly all of his days. This is Chazal's intent in (Berachoth 5a): "Let one always incite his good yetzer against his evil yetzer, etc." This "incitement" consists in his always waging war with him, as we have written.