So, you are 13/14 or you are 17/18 years old. I am guessing 13/14 since you are still looking at going back to school next year instead of taking other measures and mother's are involved.
You have a lot of problems some of your own making and others that aren't. Are you still getting help for your problems?
Obviously the issues that have Heather's Mother upset existed before you started getting help, but not before the problems began. I have a feeling that both Heather and her mother may be more aware of the affect your issues had on Heather who is probably about your age. Heather probably has her own issues from all of this and her mother is trying to protect her.
I know that is hard for you to understand. Your focus is on your own problems. Even at your age, it is time to take responsibility for what you can do to make your life better. Words do very little to show the progress you are making. Actions speak louder in cases like this. You may not be in the same school Heather is, but I bet you are either being home schooled or are in an alternative school. Which ever path you are own, do your best to make it work. Keep your grades as high as you can. Stay out of trouble. Talk to your mother about things you can get involved in that will help you manage your depression and other issues. Hobbies, diet, exercise, volunteering, etc. can give you productive ways to give yourself mental, emotional, and physical support. Be honest with your doctor(s) and counselors about what you are thinking and feeling. With your mother and doctor make certain the meds you are on are working for you.
Earn back privileges like your phone. Earn back the respect of Heather's Mother and Heather. Become the healthy friend that Heather deserves and that you deserve.
After a few months of getting yourself on track, see if Heather's Mother will agree to a supervised meeting between you and Heather (if Heather would be okay with it.)