Hey I think you should go for it!
My story is different but I can relate in many ways. I am currently finishing up my MA in Professional Writing and was recently accepted into a Ph.D. program in Rhetoric which I will begin in a few months. My MA program has been remote.
I have been incredibly fortunate to complete all my education thus far, but my BFA was in Classical Piano, completely unrelated to what my MA is in. I wouldn’t have had an academic career at all if I hadn’t received a scholarship to study piano. So I can relate to your anxiety because I went into a graduate degree that’s completely different from my BFA and with it came a complete and total career change. I took the plunge and I’m so, so happy I did.
I can’t speak for the school you were accepted into, but most of them are this way: in my experience, undergraduate programs weed students out. But graduate programs want you to succeed. You typically join a smaller class (or cohort) and you will be a student, not just a number (again this depends on your program, but it tends to be the case).
I have received such an unbelievable amount of support throughout my graduate experience. I have been very transparent about my career change and my advisors and classmates have always been willing to help me if I experienced difficulty in the program. I have had moments where I was behind because everyone else was coming in with BAs in English or coming in with not only degrees in the field but over one or two decades of professional experience whether it be teaching in public schools, conducting research, and writing for pretty serious journals. I didn’t have any of that. I just knew how to play Beethoven really well and I knew how to write fiction, which was completely unrelated to the kind of writing my MA is in (although it’s benefitted in so many ways as a student that I can’t count them). But because of the support I’ve received and because I’ve been working really hard, I was able to “catch up” pretty quickly and I have been maintaining a 4.0 GPA with several publications in the works. I was expecting to barely scrape by, but I’ve been doing really well despite the rigor and unfamiliarity of my program.
My classmates, professors, and department have been so patient with me. I have so enjoyed my program and I have enjoyed watching my skills flourish. The work has never been too overwhelming but challenging enough to keep me engaged. I am excited about my future whereas before, I was feeling pretty hopeless. COVID killed my career as a musician and having to make a complete 180 with my life was so daunting, but I’m so glad I did it.
My program is a year and a half total, but the length of your program will depend on your school, department, and your personal life…like if you are going full time or not. Three courses a semester is considered to be full time. Two or less is part time. A master’s program is typically two years. Library science is typically two years (my department works closely with the MLS students so I know a good bit about the kind of program you are doing). The timeline for American higher education for full-time students is usually 4 years for undergraduate, 2 years for master’s, and 4-7 years for Ph.D. You do not need a Ph.D. to be a library scientist. Your field is incredibly lucrative and you have chosen a fantastic career path. There are lots of great jobs in library science that an MLS will award you, and these are the kind of jobs that will take care of you for life.
You may or may not have to write a thesis. Things are changing and more schools and programs are offering elective tracks where instead of writing a graduate thesis, you diversify and take more courses. This is because employers would rather see diversity in a CV than time spent writing a long academic paper–but this is definitely on a case by case basis. For my field, my employers want to see diversity (yes, I can do grant writing, teach literacy, conduct research, utilize databases, work on databases, draft professional documents–legal, scientific, and engineering, all because I took elective courses rather than writing a thesis. This makes me more competitive in my field). But it depends on the field, the program, and the department.
Sorry for the long post, but I 100% support whatever decision you make! However, I also don’t think you need to be so anxious about it because I think you’re going to get into it like I did and realize that it’s not that scary at all, that you can do this, and that you can do it really well.
I hope this helps!
then again, idk much about Masters degrees 
But good to know and good question to ask!
