I love him, but why can't I see myself growing old with him?

I can't bear losing him. He has my heart, I know he does. He's the only guy I've ever loved this much. In all honesty I even think he's the first guy I've truly loved. I hate the fact that I feel this way, that I can't imagine spending the rest of my life with him. I dont know why, when I know losing him would hurt the most.
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I don't know what to do. it doesn't feel right to break up with him but again it seems I can't imagine growing old with him. its gonna hurt when he leaves. do I break up with him? I don't know wtf.
I love him, but why can't I see myself growing old with him?
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