I think it’s too early to tell if you’re cut out for grad school, but there are a few things to consider.
Graduate school is really not like college. It’s quite stressful and competitive even if you have always “loved school.” Once you give up the idea that it should be fun and intellectually rewarding, like college, it becomes paradoxically a lot easier. Treat it like a job, with parts you like and parts you don’t like. The process really isn’t about enrichment and finding yourself. It’s essentially trade school for professors. Remember Sayre’s law: l"Academic politics is the most vicious and bitter form of politics, because the stakes are so low."
I think you are correct to be concerned about your grades. In our grad program, a B as a course grade is considered a warning sign that someone might not be cut out for the program. However, a B on an individual assignment during the semester is not disastrous. The grading scale is more compressed than undergrad. As @apprenticeprof pointed out, context matters. What does a B mean in your particular program?
How rigorous were the standards of admission to your MA program? Is it a full-time program? Do you have funding?How many people from your program actually go on to get PhDs? Most of our job candidates in English skipped the MA and went directly into funded full-time doctoral programs after undergrad. The MA only qualifies you for adjunct positions, which are unstable and pay close to nothing. The good institutional funding at universities with PhD programs in English generally goes to students who started their graduate work there, not on those coming in with MAs from elsewhere.
I would love to say, “follow your dreams and don’t give up,” but I also feel responsible as a member of the profession to point out that there are very few jobs, placement failure is exceptionally high, and an MA is not a particularly valuable degree in the profession. You will also need to be willing to move around. Searches for full-time positions are national in scope. If this MA is costing you money or requiring you to take out loans, I would think long and hard at the end of the semester about whether it’s worth it. It probably isn’t, at least not from an ROI point of view.
If your class is like most grad classes, your course grade will be based primarily on your research paper, which you After that is done and you’ve received the course grade, you can reevaluate whether the MA program is for you.
If it’s not, that is okay. I have known several graduate students who made the decision to change course and ended up relieved and at peace with their decision. Make sure that your goals (what you want out of the program) are realistic and attainable. Look hard at your program’s placement record. Good luck to you.