Should I let a bad mark from 10 years ago discourage me from taking on a TAship?
This is ultimately a personal decision for you to make and it's likely that the question will be closed.
I assume that you now have adequate background to be a TA for this course, so you've overcome the difficulties that you had when you first took the course.  Let me suggest to you that your experience in struggling with this course as a beginner is something that can help you in working as TA for the course because you'll understand how the students are sometimes struggling with the course.