I know it's really tough to talk to people about this! You mentioned that you don't want people to think wrong of you, or to think you want attention...I understand that feeling. It is hard to ask for, and rely on others for help. Could you re-visit the topic with your Mom? And let her know that you said you didn't have a problem at first, but really you were afraid to mention it for the reasons you named?
It sounds like your Mom is aware there may be something going on and she is concerned, and would help you.
If you can't talk to your Mom, is there someone at school, like a school counselor who could guide you?
From my own experience, I know that eating disorders are difficult to face, and take time to get over. I also got scared when I would feel heart palpitations. Because I was depriving myself of nutrition for years, I too had trouble paying attention and cognitive problems developed. It seems like you are aware that this is happening to you. At 15, I want to tell you that your brain is still developing. This is an especially hard time and I want you to know that it will pass. Not overnight of course, but it will and I want you believe that.
For now, just be kind to yourself and ask if your Mom, or someone at school can set you up with a councilor. Try to have foods like Salmon, or other fish, or take Omega 3 vitamins. These are good for your brain and your heart.
I think it's important to tell the Dr. who put you on antidepressants because I think some antidepressants can make these problems worse sometimes. They did for me at first! Also, on your road to recovery, you will have relapses. If you tell yourself "It's the last time," don't be hard on yourself if it turns out not to be.
If you have any specific questions, please ask and I will be honest and open with you.