I would try and tell him the truth - talk until you're blue in the face if thats what it takes for him to get that you are serious.
It's really hard, contemplating changing your life, but I believe that you shouldn't stay married if it isn't right for you. Obviously put a lot of thought into it, but I believe it is ok to change your mind.
I also don't think marriage is a realistic thing. I too married young, and if I was able to do it all over again I wouldn't. Not because I don't like my husband at all, just because I don't think it is realistic or necessary.
Good luck to you. Start with the truth. It's not always the easiest, but it is the best!
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