I think your essay could be as follows:
Two years ago, I had a very close friend. I used to consider her a really trustworthy person and I never kept any secrets from her. One time, I told her about this boy that I liked, hoping that she would appreciate my willingness to share things with her. However, after having heard this, she ceased being friendly towards me. In fact, she began to pick on me in public. At fist, such her behavior did not make any sense to me. However, it was not long before I found out that this girl had a crush on the same boy. The lesson that I learned from the incident is quite apparent - do not become too close with even the most loyal of your friends, because there is always a chance for them to turn into enemies while competing for the same 'resource' with you.
I hope this will help. Regards.