Is it permissible to make up a day of fasting that I missed after deliberately breaking my fast, then repented and fasted the days I had missed, but forgot to fast one of those days?

You must make up the day you missed from Ramadan, for Allah Almighty says: {a number of other days}. If the delay in making up the fast was intentional until the next Ramadan began, then a minor expiation is obligatory upon you, which is feeding one poor person for each day. However, if the delay was due to ignorance, forgetfulness, or an excuse, then only making up the fast is required of you.