Chipper Monkey is right. To break this cycle, you have to stop becoming friends first. When you are attracted to a woman, let her know that you like her in a romantic (not platonic) way as soon as possible, before you head down the road of platonic friendship.

When a woman rejects that romantic interest, do not develop a platonic friendship with her in order to change her mind. Personally, I would take a six month break from the person to get over the crush before attempting a friendship (if I still wanted one at that point).