Short answer: Live in reality.
I don't get oneitis anymore. The closest thing I get is higher than normal attraction for a woman.
Its hard for me to get onetitis when I don't respect or trust women enough to see them in such an elevated light that I would pedestalize them. I see really hot women as higher value because of the arm candy and the increased sexual desire. On the emotional level.. they don't do anything more for me than average women. So no oneitis.
Get more experience. Once you see women for what they really are.. you will realize they are just flawed humans that are not worthy of being worshiped or idolized in anyway. When a man has experienced years of rejection and some cute girl comes along and treats him better than normal.. that is all it takes for him to get oneitis. Its kind of pathetic. And because the man is so starved for love he is blind to the fact that all women have agendas and her behavior is more about HER and what she wants than about him.
When you understand this game on a deep level you don't get oneitis. You also learn not to cling to things in general as life is unpredictable and change is constant.