At 15, I believe that you are mature enough to know the facts surrounding your birth and adoption. By keeping the facts from you, she is only making the matter worse. You must try to understand that your mother, the only mother that you have known, is probably afraid that you will want to leave her if you find out the truth. She already knows that you hate the fact that she has lied to you. This is a very serious, complicated situation and I am sure that you might be able to place yourself in your mother's position and understand why she feels as she does.
There are therapists who are specialists in helping kids in your position. Perhaps another family member can help you to persuade your mother to see such a therapist so you can both begin to discuss this issue without blame or anger. No one can make your mother tell you anything. But if she felt that she could help you by telling you the story that you need to hear, maybe she would feel differently. Please keep me up to date on what happens.