It's great that you finally broke things off with your bf and told the guidance counselor at school. Great job!
I assume that it is painful to see your ex now. It's difficult not to personalize the sad look that you always see on his face. But remind yourself that his sorrow isn't your fault, we can't always rely upon others to give us the motivation to live. We can only control our own behavior, no matter how hard we try!
I'm sorry that your mom is having you change schools, to improve your grades. It is hard to let go of our comfortable space and be put into a new one. I wouldn't be real happy with that decision either.
Are you thinking that if you tell your mom why you've haven't done as well expects from you that she will change her mind about making you switch schools? If so, I'm not sure that's real logical. I suppose that it depends upon how the information is presented to her, and her personality style. {Some people are real "hotheads" and blow up; others sit and think for a while before reacting; a lot of people react pretty well in the moment, before the information sinks in and actually gets processed ~ then they blow up later, over seemingly nothing; etc.} Anyway, you certainly know her style by now. I just recommend that you think about how she typically reacts to stress and go from there.
Many parents are very supportive and understanding with their gay children. Hopefully, your mom will fall into that category when you do decide to let her know.
Best wishes to you!