Siman 530

(87.) Once he spoke with me about distancing the general desire — which is the desire of immorality — and he greatly mocked this desire as was his way. He answered and said: "It has always been difficult for me — what our Rabbis of blessed memory said [Nedarim 20b]: 'One should regard it as if a demon had compelled him.' Who compels you? Meaning: if a person wants to completely conquer his desire — there is no one who compels him. And understand."