I drank early this morning but realized it was too late after the zman
Someone that mistakenly drank or ate during the fast, should continue fasting the rest of the day. Regarding saying aneinu, he can’t say it because he did eat during the day. Additonally if you only drank less than a cheek full, although we are not allowed to do this, however b’dieved you would still be considered as fasting.
O:CH 568-1, M:B 568-5,8.