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Why do you need a cure? Is it damaging your relationship severely, whatever you're doing? If not, then be yourself, let yourself change naturally, if at all.

Did you give up smoking and join the gym for you or for him? Does he do everything he can for you too? In what way do you 'muck things up'?

It sounds like you're changing away from who you are to what you think you should be and/or what he wants, particularly as you say that you muck things up by, presumably, being who you are.

Maybe that's a little harsh, or I've misinterpreted the information in your post, but he doesn't exactly sound like a perfect match if you have to change yourself for him.

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It seems, from what you've said, that you make the kind of daft faux pas that everyone, even those in the most long-term of relationships, can make.

This is not to diminish your concerns, I can tell you're worried. But think...does it have a significant adverse affect on your relationship? If not, then he seems like a cool guy, someone who can take these silly little mistakes and laugh at them afterwards. :)

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...and now I am pretty sure that he thinks that I am cheating on him with this guy. ...
One or both of you sound pretty insecure if that's the case. Has he told you directly that he thinks you're cheating on him with this bloke, or is it just your assumption based on his behaviour and/or hints?

You need to sort that out before it turns into a major insecurity/jealousy issue.

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