The /d/ is not fully exploded in this expression, but it is usually there. Having said that, I have to damit that some native speakers do just say 'I better'. Some even write it.
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jimmysaenz what's the negative form? I'd better not go ? is it correct?Yes
English School Group It is already negative. we may write it like " I'd not better go"That is not possible in BrE, and there are no citations in the Crpus of Contemporary American English.