Eventually, you just learn to look at the present and the future, and not let the past over take you. We all have pasts, and you really can't change it -- you can only go forward and make your own future.
You're 21, and you have plenty of life ahead of you. Be glad that you can start now, and not wait until you're older. There's a lot of people in here who are older and just starting out. The good thing is that things are different now than they used to be... mostly with information and access to doctors who are informed and will do something.
As for my past, I came out when I was 5 or so. I was sent to psych doc who viewed that I could be be cured via forcing me to be a boy and pushing boy-centered activities on me. This was in the early '80s, in a small town. It was very popular back then that if a kid shows signs of GID or being Gay or Lesbian, that those things could be "cured" via activities. Of course, that was proven not to work in countless of cases and not adopted as a method anymore by main-stream psychiatry.
I went through that as a kid. Then as a teen getting beaten up at home and school for it. Then going through church councelling to make me un-transgendered. Then going through ex-gay therapy at 19 via church. Then being really screwed up mentally from all of that. Trying again at 21-22, but flopping 'cause I had no support and had no where to go or information at all, plus my family saying no (my mom sorta supported by then, but wanted me to wait until my grandparents were at least dead, and warned that 99% of the family will ditch me). I then went through a huge depression, then through a lot of counseling and medications to fix that and to work with my past so I could move on. Tried school as a last-ditch attempt to see if "maybe if I got a career and a girlfriend the GID will stop." Of course, school and girl friends didn't make the GID stop, so on my 26th b-day I announced that I was transitioning and that was that. I worked hard to find support, internet connections, friends who were supportive, and doctors. Started FT and HRT 6 months after my b-day, and never looked back.
Most of my family doesn't talk to me. My grandparents aren't dead, but view me as dead. My brother, who I used to be extremely close to, no longer talks to me. My dad, rarely. My mom (dad and mom divorced when I was 2) is the only one in the family who talks to me and accepts me. I'm (ugh!) 31.
It hurts. Even after all of this time. It hurts that my family still refuses to talk to me. It hurts that I went through bad-therapy several times over while growing up, via out-dated psychiatry and church.
It's not easy. I would love to time travel and start earlier, but I can't. The past has many painful moments -- but the only thing I can do is hold my head up high and choose my future. I don't think the pain of the past will ever completely go away. I'm sure that I'll still wonder what it would have been like if at 5 my GID was taken care of correctly. But, I can't live off what-if's, and I can't let the pain from the past drag down my future. It's not easy, but you just needa keep marching on in the direction that is right for you.
Anyway, good luck with everything. *hugs*