Online high school is stressing me out. Please help me?

I am a junior in High School. During my sophomore year, I went through a ton of crap that convinced me to quit public school. I was switched into online schooling for this year, and I thought it would be a lot better. But turns out it's not. It's the end of the semester, and things have gone downhill quickly. I'm behind in three of my five classes, because I wanted to take my time with some assignments and of course sick days. I want to finish, but every time I log in I see disheartening messages reminding me that I MUST finish or I WILL be failed. I end up getting nervous and thinking that I'm pretty much screwed and will fail. I try to catch up, but I feel that it's just not worth trying anymore.

Most of the assignments are simple, but I just don't feel like doing them. The others are DBAs, or Discussion Based Assessments, where I call the teacher and we do a lesson review. My problem with these is that I have social anxiety (Part of the reason I wanted to switch to virtual school in the first place), and calling the teachers makes me nervous. I'm worried that I'll get chewed out for being behind pace, that I won't know the answers to the questions they ask, etc. I end up cancelling the DBAs an hour before I'm set to do them (Or I purposely don't schedule them). Now I need them to catch up, but they're all booked up, and the ones I can get I end up cancelling/skipping because of nervousness.

I feel hopeless right now, and the semester's almost over. What can I do?