Falling on your sword is a very Roman thing to do. Your professor might appreciate that. Just make sure that you have no other alternatives, lest you find yourself in Antony’s situation after Actium.
Studying vocabulary: use each word in a sentence, or think of a sentence from your reading that uses that word, to remind yourself of the meaning. Write down the whole declension of nouns and adjectives, or the whole declension of verbs, and read them over to yourself; if you only remember two of the cases, or two of the forms in two different tenses or moods that’s still enough for you to remember what declension or conjugation the word is.
Studying grammar: chant inflections. I still remember -bo, -bis, -bit, -bimis, -bitis, -bunt as one that stuck with me even after I’d internalized a lot of the other grammar. Every time you learn a new grammatical construction, write a few sentences that use it.
This isn’t something you can cram. This is probably a learning experience. I hope your grade can recover.