For a start, don't panic. Even if you have to wait until your 18, you're really only missing out on height. Which, chances are, may not be affected by hormones anyway. Most of the heavily masculinizing changes associated with puberty actually don't occur until the end of it when there's nothing level to do with the testes, scrotum, and actual phallus itself. So . breathe .

Next, step back. Address the issue. Dad, grandma, why would you want to hold back on this? Do I come off as immature? What makes you doubt me?

Try to find compromise. Ask for blockers instead, those will compromise secondary sex characteristics, but they won't play with your health, bones, or growth short-term. You could definitely use these until you are 18 to no downside, and resume normal puberty once you're off, or go to hormones and induce feminine puberty. Try to "barter" for that, instead of actual transition.

Ask for a therapist, and when asking, don't ask "so I can transition," ask "so I can figure this all out and see if transition is or isn't in my best interests." A therapist who specializes in trans patients is what you would need. Preferably someone who is trans, they won't favor you. They'll just recognize key signs that they themselves have experienced.

And don't jump to conclusions. You are young. Hell, many people under 20 find that they leap too far when going into this. It is a legitimate concern, and I don't believe that even 18 year olds are fully mature enough to make the decision to sterilize themselves. So explore. See if there are other issues causing or interfering with this, see if this isn't something you're scapegoating from other problems. And if it's determined that you aren't (it may be a few months, but a few months won't hurt you, I promise promise promise!), then there would be professional proof and opinion that indeed you need to transition. Blockers can still be used before all of this though.

I started at 15, I've gotten through to my family and I'm already on hormones. If you need to, PM me, have your grandma or dad PM me, I'm willing to talk to them if that's something you need help with and are comfortable with. Good luck!