How do I forget what this friend did and get on with the present?

That 'friend' of yours is no friend! This is a very crappy situation for you.

Just know that your grades won't matter once you graduate from school, all that will matter is that you have a degree in whatever you are pursuing. Nobody checks your grades when you apply for a job.

This person you talk about reminds me of one of my high school friends. We used to phone each other and compare our homework every night. She would always take my answers and correct her work or add my ideas to her answers. When I was wrong on an answer she never corrected me, she would just say she had the same answer as me. The next day when we got our stuff marked she would miraculously get 100 percent on the assignment and would get a few wrong and she would brag to me about it. It made me so mad because she was my 'friend' and I helped her get 100% and she would just use me and toss me to the curb. It wasn't cool. Believe it or not, this crap went on for a whole semester until finally I cut her off as a friend. She had good grades but she also lost all of her friends so I'm pretty sure I'm the real winner in this situation. Grades only matter while in school. Afterwards they mean nothing (unless you don't pass).

Good luck! I feel for you!

By realizing anyone who lies isn't a real friend to begin with .
Move on and stop comparing yourself to others.
Doing the best you can should be worthwhile enough.

Focus. You need focus. And more time.

Do you see the friend. If not then just forget about him/her