How do I tell my ex that I found someone else...?

Me and my boyfriend have been going out for 4 months. Through this, we were perfect for each other. We would always go out or spend time time together. Apart from that, He cared for me and supported me in all the times I needed someone. He would tell me I'm beautiful, no matter how much of a mess I looked. And most importantly, he made me feel special; like I was some kind of girl that made him happy. All in all, we both knew we had something right in our relationship. A few weeks ago, I was heading out of the spa to Starbucks. Then I notice my boyfriend at a table talking to a girl. They didn't know I was there so I tried not to interrupt their little chat. But I grew kind of suspicious when 'this girl' was flirting with him and so was he. Though I thought they were friends so I let them be. The next day, I see her again at his locker and they were doing the same thing as before. Later on, my boyfriend brings me to the corner and says we weren't meant to be (even though we were fine!) He was hinting a little bit about 'finding someone else' so I had assumed he meant that girl. I was horrified and I couldn't help but be upset since I thought we were close. I told my guy friend all of what was happening and surprisingly made me feel better. He then admitted that he liked me ever since we met and hoped we could date. From that day on, we just flirted and I wanted to be with him. And coincidently, my ex comes running back for me! He had apologized and tried winning me back.