Without ever being able to talk with your mom about why she ran away, you are left with no understanding of why she abandoned you. I am sure that others have given you their opinions of why she left but the only one who really knows why is your mother. It's possible that your mom ran away from her life, not from you. She may have become so overwhelmed emotionally with some of the things in her life that the only way she felt that she could survive was to run away from those things. I don't know what those things were and neither do you. But I do not think that she ran away from you or that she didn't love you.
I am not excusing what she did and I am not asking you to forgive her now. Just consider the possibility that leaving you was the hardest thing that she ever did and that she may regret it. I am sure that she must still think that you are better off not having her in your everyday life.
You are right; the time for you to talk with someone about all these unsettling feelings is long overdue. Please ask your grandmother, a trusted adult family member or friend, your school counselor, a religious leader, or your regular doctor for the names of therapists who can help you with these difficult feelings. Many therapists often leave room in their schedules to see patients who would have difficulty paying for free.
There are good people out there who can and who will help you feel better about your life and help you understand why this happened to you. Thanks so much for writing.