Sorry you're feeling so down.

I'm not sure what the age of majority is where you are but you're certainly an adult by my reckoning. As others have said, that means it's time to start figuring out how to live your life as an adult.
Easier said than done, eh?
The transition between childhood and adulthood is a difficult one; I would say it's on par with the transition from female to male. It takes
years for you to establish yourself as an adult, and it's done in small, incremental stages. Just as it takes years to fully establish yourself as a man.

Here's the thing: when you transition from f to m, nobody tells you exactly how to be a man; you have to figure it out for yourself. Sure, there are plenty of resources out there that you can learn from, but there's nothing quite like practical experience to help you figure out what kind of man you are. Likewise,
nobody fills you in on how to be an adult. Nobody gives you permission to start doing things your own way... especially not your folks, who have their own vested interests in keeping you under their thumb. So do you want to know how to adult? There's only one way to do it: you have to
take your independence, because nobody's going to give it to you. You start with little things, and you do them one at a time. And each one you succeed at doing will make you feel slightly better about yourself. Eventually, that will add up to you feeling a lot more confident, and at that point you can tackle the big things.
So there are quite a few little things you could start with:
- Go back to your original therapist or find a new one - but this time, book your own appointment. You are not a child any more and you don't need your mother's permission to see a therapist, or any other professional you choose. You could ask for a phone consultation if you have trouble getting to their office. As you are an adult your family has no right to attend the appointment. Tell your therapist that you want the sessions to be confidential and you don't want your family there because they're abusive towards you. That way, if your folks insist on coming with you, you can get your therapist to tell them to wait in the waiting room.

- Look into colleges and make a decision about which one you want to attend, and what you'd like to study. If your folks are still saving up, that's great - but look into scholarships and lower-cost courses if there's any chance your folks might refuse to fund you. Alternatively, look for a job and earn your way out of this situation.
- Do everything in your power to transition as much as possible whilst waiting for hormones. If you haven't already done so, cut your hair; buy a binder; use a packer; throw out any female clothes and replace them with male clothes; change your toiletries; stop shaving; do some weight training etc.
- You could practise Gandhi's principle of 'passive resistance': simply refuse to play along with your family's nonsense. E.g. don't answer to your birth name; only respond to the correct pronouns; refuse to accept anything feminine that is bought for you; and if your grandmother starts calling you 'young lady', interrupt her and say "Let me stop you right there: I'm a young man and I am not prepared to be called a 'young lady'."
- You could stop looking to the past, and start looking to the future. Sure, you've been waiting 20 years to be the authentic you, and it's incredibly painful to look in the mirror and see the wrong image every day. But instead of thinking of the years gone by, think about (and plan for) the next few years ahead: by the time you're 25, you'll have moved out of home and may even have completed college. You might be working in whatever field you choose. And you'll be in the process of transitioning, I guarantee it. Picture yourself at 25, and hold on to that thought. It will become your reality. Do you really think your folks will still be getting away with this nonsense when you're a 25-year-old man? Or when you're 30? or 40? Of course they won't!

- You have selected a name for yourself: so use it wherever you can. Start with places like Starbucks etc. (where they ask for your name when you order); get your friends and perhaps your therapist to use it. Then when you feel more confident, insist that your family starts using it too.
- Spend as much time as possible with people who validate you, even if it's just us lot. And find a local LGBT youth group that you can join so that you can be around people who understand & support you. That'll make your family's current non-acceptance easier to cope with, and you may pick up some useful tips to help you cope. In time, your folks should come around. But even if they don't, there will be other people who love you.
Scary stuff? You're right, it is: this is the reality of becoming an adult. But it gets easier with time, and each small success will make things that little bit better for you. And in a couple of years' time, when you've been on T for a while and are fully established in your male identity, your family will look like idiots if they try to refer to you as female.
If your family doesn't support you, do as much stuff as you can away from them. You're right to not talk about it in front of them at the moment, if that's going to start arguments: just keep pushing forward in the right direction and they'll be forced to catch up eventually. My family is also very unsupportive, so I've learned to not rely on them for support. I'll stand on my own two feet if I have to.