One of the most memorable times this happened to me was in high school. I was dating a guy who seemed really sweet and genuine, but after a few weeks of dating, I found out that he had been seeing another girl at the same time. He had been telling her the same things he was telling me, and had even taken her on the same dates he had taken me on. I felt so foolish for not seeing it sooner and for believing everything he had said to me.
It was definitely a tough lesson to learn, but it taught me to be more careful with who I trust and to always keep my guard up until someone has proven themselves to be trustworthy. It also helped me to realize that I deserve someone who is truly committed to me and who values me for who I am.