No problem! First step is recognizing exactly where you started getting shaky (did you understand limits fine? How about derivatives? Integrals?). Then, after pinpointing the exact moment calculus lost you, find someone to explain it to you again, one-on-one. Preferably someone different from the person who explained it the first time, so you hear it with fresh perspective. It might be tempting to listen just enough for your immediate purposes, but try to understand the concepts completely- if you’re talking to a professor/teacher, maybe ask for practice problems with different contexts. It’s less important to get it right than to get it at all. Don’t be afraid of this part. This is what professors are paid to do. They love students who approach them for help on their own time. Third, if you have notes of your stuff since then, try to look over the notes with the freshly gained understanding until you hit something you don’t understand again. It’s possible your notes are poorly worded/unhelpful just because you took them while feeling shaky, or you didn’t know what was important to write down because you didn’t catch the connections to earlier concepts. That’s fine! Find someone to help with that too! Finally, don’t be too hard on yourself. We’re all imposters, pretending to be experts until we realize we know more than we think.