Do I leave him?

My so is my best friend... We have been together for almost 2 years. Our relationship has been rocky from the start, and we've broken up a lot but always get back together. I love him and I want to spend the rest of my life with him... But in December we broke up and I was heartbroken, he was still my best friend and the person I loved, we hung out almost everyday and texted everyday but we both went out with other people. I met this new guy who was amazing but is leaving in 2 months for at least 6 months. Well my so said he was going to change and wanted to try again, so I dropped the new guy.... But now I'm not so sure about being with my so, don't get me wrong I love him and want to spend the rest of my life with him, but something is off and my mind keeps fluttering back to the new guy... I don't know what to do... I realized how bad it was last night when we were having sex and my mind went back to the new guy and I had to stop it right then and there... I love my so and I want to do what's right for him because he is my best friend... And in this point in time I don't think I'm right for him if I can't even stop thinking about a guy I saw for less than a month before. My so says if I break up with him or ask to just be friends he will just leave and never talk to me again, which I can't handle because he is my best friend and I love him. I just don't know if I'm in love with him anymore. I don't know what to do, please help... I don't want to lose him