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"More forgetful" is the comparative form of forgetful.
"Most forgetful" is the superlative form.
There is no such word as "forgetfuller".
Most forgetful.
He is a forgetful person as he forgets his wife's birthday. Forgetful is a word to show not remembering.
The opposite of forgetful is mindful,
EX:
I can be vary forgetful sometimes.Sorry I forgot I'm very forgetful