Mayor Mare,
I take it you like horses. I can't tell you what to do, but sometimes what our parents can't accept in others they will accept in their own child. If you haven't already, do as much research as you can. Not only on transitioning, but theories as to why it happens. You will have to sort through some papers that are biased and untrue, but stick to the ones that show transgenders in that it wasn't a choice, but rather something that went wrong in utero. Make sure not to suggest that your mother did anything wrong during gestation, but something like a neurologic anomaly.
If you can't do that, you might want to go someplace where no one knows you an pass. If nothing else, that may give you some relief. If you haven't started growing facial hair yet, you can probably do it.
Remember, even if you do it at 18, your parents are going to go through the stages of grief. Finding out that your child is a girl instead of the boy they thought they had is very much like losing a son. They may be angry, depressed, guilty, the whole realm or just some of the stages. Give them time to work it out and try to be patient with them.
Stay strong.
sam1234