I'm sorry about your news. I suspect when the doc got some cells, he got them from a section that wasn't as advanced. Then the next time, he got them from an area more advanced. As you say, I also find it hard to believe that they would get more advanced over that short period of time.
I assume they will be able to get the bad cells out. I know you are worried and afraid for the future, but as long as things haven't gotten into other organs, you have a very good chance of being around for many years.
I know it will be hard to do, but try to concentrate on other things, such as school. Worrying won't help and will make your life more miserable.
Is there a counselor the doctor can recommend to give you support?