I'll just add to what's been written below: On line school didn't work for you, obviously. You only managed when your sister was there - so you weren't doing it on your own. It was, at some point, your obligation to inform your mother that you were struggling. But it also, in my view, was your mother's obligation to check in with you and with your teachers, and your teachers' obligation to send progress reports and suggestions for improvement. So no one was communicating with anyone else. There's fault on all sides. So you did not fail her. All aspects of this system failed.
So you sit down and tell her that you need her help. It's not the end of the world. You can make up the work or, better, find a better program that gives you more direct interaction with your teachers.
Moreover, you and your mother need to look at the reasons for the move and the tremendous adjustment that was for you. It sounds like you flourished in a more involved learning community with friends, activities and teachers vs. a virtual relationship with only a computer. That's a lesson learned - you need the community around you.
If you are close with her, what is it that scares you about what she will do? What kind of mom abandons or rejects a child who needed help and didn't get it? If you are close, she will value that as much as you do. Tell her what you need - her support, her love, her commitment and her help to get you into a better educational situation.