How do I tell my ex things are serious with me and a new guy?

My ex and I broke up not that long ago and I started dating casually. I ended up meeting someone who wants to be exclusive and had the same serious intentions I do. My ex and I were in an agreement to uhh... keep each other company casually until we found someone else. I didn't think this would happen so soon but how and when do I let him know it's done? I know he may not want to be friends and that it'll shift the dynamic a lot. How do I cope with missing him even though I may have a new amazing boyfriend soon?

How do I tell my ex things are serious with me and a new guy?
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