Hi.
I can understand your confusion about your mother's advice to you versus her own behavior. If she issued a warning not to have sex before marriage, she may have meant "Don't have sex until you are old enough to accept the potential consequences that come along with sex"--namely, the risk of pregnancy and sexually transmitted diseases and also the responsibility to prevent such things. She may also have been worried that you were not old enough to handle the emotional ups-and-downs that may accompany relationships in which physical intimacy exists.
Your mother is an adult who has already had the experience of child-rearing so it probably feels like, "been there/done that" to her. On the other hand, you are a teenager whose main focus right now is probably school. Thus, the seeming "double standard."
I imagine that it was your mother's concern for you that made her issue such firm declarations. Parents aren't perfect. Sometimes they have a fear--like you getting pregnant before you're ready--but don't know how to discuss the issues involved, so they say what they think they should say or just repeat what their parents told them. I don't know if this is the case with your mother, but it may be.
It seems like this is really bothering you so much that it's interfering with your relationship with her, so you could try talking to your mom about this. I would suggest addressing your confusion and disappointment in a calm manner, though. Not many people respond well to being attacked. If you don't feel able to do it alone, you might ask for the help of a school counselor or other trusted adult.
