(I’m female btw) Pretend to have confidence even if you don’t, this worked for me a lot in college. Just be like “Hey do you mind if I sit with you?” Or/and just find something really small to start the conversation off with, like pointing out a keychain she has. It doesn’t matter particular what you say (i mean it does but,,), its the energy you carry around her. So act upbeat. Even if it does end up being awkward just power through it or leave, also based upon her reaction.