Technically, I am no guy with a major or seasoned academical, anyway, as you didn´t the degree of rustiness of your English (deteriorated/got worsen with time/can´t speak it well/you can´t speak an iota of it anymore...)
First thing in these cases is, use what you know to learn what you don´t, use the basics you learned "how are you? could you explain that/the (subject/matter here - for example "the story of English" to us? use a smile, be polite and this will do wonders for you. When you make a blunder, explain that your English is not what it was once, etc.
Train listening, how about getting some Speak Up magazines for training? or some other listening thing (tapes, videos etc), a good idea would be to watch videos on the subject you are studying at the University.
Try to train with classmates as well, since they (or some of them) will be taking the same classes with that English-speaking teacher, so the difficulty can be the same to them. And sometimes two heads (or more) are better than one.
And remember Peter advice here:I am telling you,wether or not you live in an English-speaking country ,you must burn the midnight oil every single darn night to get a hang of this untameable language.https://www.eslpod.com/eslpod_blog/2009 ... -once-was/
And here some advice from Redseahorse, it might come in handy.como-passar-de-intermediario-a-fluente-t45974.html#p180932
Hope it helps.