IPA: //ˈpeɪn.fəl.ər//
KK: /ˈpeɪnˌfʊlər/
More intense or severe in causing discomfort or suffering than something else.
The injury was painfuller than I had expected.
Comparative: painfuller
Superlative: painfullest
'Pain' (from Old French *peine*, meaning suffering or discomfort) and '-fuller' (a comparative suffix indicating more of a quality). The word 'painfuller' means more full of pain or suffering than something else.
Think of 'pain' as suffering and '-fuller' as indicating more of that suffering — so 'painfuller' means experiencing more pain.
No commonly confused words.