I don't have the courage to break up with him...

We've only been together for 9 months, but the relationship is negatively effecting me. He still cares about another person more than me, and Everytime I see then together or talking it brings me to tears. The way they talk about each other just cuts me up inside. (How much they still care about each other). I honestly don't know if I can trust him. He's cheated in the past but it wasn't anything too bad so we moved on and I forgave him. I just don't feel I can trust him anymore especially with everything that's been going on. But I'm so attached and so in love with this guy that I can't let go. I don't want to live without him. Help me, I need support.