"That's not going to happen."

We dated for a few months in high school, and a few years later we attempted to rekindle. He and I met up and talked for hours then later kissed that night.

We never really ended on bad terms, but during our rekindling period, which went on for a few months, we were hanging out by ourselves and I didn't want to do anything with him because I was worried, and told him, "I am scared this is all just going to end after we hook up and that you'll go back to your other ex." He reassured me and said, "That's not going to happen." So we proceeded to keep going that night.

A few weeks later, he is now back with his ex and I just feel broken for not following my gut instinct.