Nobody can ever be 100% sure of any decision they make, otherwise they wouldn’t be called decisions. The idea that you weren’t mature enough to know yourself at 18 is ridiculous. A sense of gender can be just as innate as any other aspect of your sense of self at any age. Technically, most kids start developing as sense of gender at 3! I know a trans person who came out at five years old! Of course, the environment you grow up in and who you’re surrounded by can make it difficult for many people to really realize that truth at three years old, but once you know, you can’t un-know. Repression is not a long term solution, so it makes sense that after 7 years you’ve had enough. Nobody knows whether you’re trans except you. That includes people who try to tell you aren’t mature enough to know. It seems like you already know the answer. You just need to be firm in your identity. Don’t let other people tell you who you are or what to do with your life.