Chazal tell us that a desirous thought for a woman is "worse" than committing the illicit act (Yoma 29a). This means that thoughts of desire for a woman are a greater pleasure for a man than the illicit act, itself. What's more, since a thought is private, it is hard to motivate oneself to do teshuvah (i.e., repent) for it. In contrast, an actual improper action is tangible and (possibly) known, so it serves as a powerful impetus towards teshuvah. So in terms of spiritual regression, the thought is worse.
Let's look at the process more deeply. When a man looks with desire at a woman (who is not permitted to him), it causes spiritual impurity. But if a man cuts off the thought, the process reverses the negative streak and becomes the genesis of four good things:
(1) He protects his mind from tumah.
(2) He stops the inclination to commit an aveirah.
(3) He initiates good thoughts.
(4) He performs more and more good deeds because - as pointed out above - "One mitzvah leads to another mitzvah."
Cutting off the improper thought at its inception is known for this reason as "the root of all goodness." Even if a person is caught in a chain of improper thoughts, he can break the chain by cutting off his thoughts just once. Spiritual levels ascend progressively. The merit of controlling your eyes even one time will empower you to control your thoughts in the future and reach successive levels of kedushah.
Today: Remember that by cutting off improper thoughts just once, you begin a new habit of control.