From where I've been and what I know, it hurts because you know what they want. They want you to grow up and be a happy man with his wife and family and all that stuff. It hurts because you know that you can't ever live up to that expectation. It hurts because you know that's what *they* want but it's not what *you* want. You don't want to grow up and be a guy. You want to grow and mature into the woman you know you are.
That conflict hurts like hell. I've been there. I cannot tell you how many times my family told me what a good man I was becoming as I got older. It made me uncomfortable. It hurt. It felt like they were reveling in the thing that I wanted least in the world and it hurt because I knew it to be the lie that it was. I knew that I was a good person. I knew that I was considerate and kind. I knew that I wanted love and happiness but I also knew that I couldn't find that in the way that they wanted me to. It's the expectations of others pressing onto our own needs and desires that causes conflict and, right now, you're the only one aware of that conflict.
You will have to deal with a lot of things here. First, you're going to have to come to grips with who you are. You're you. You can't be what others want you to be. You can just be you. Period. Done. Nothing else to say here.
What Murbella said above is true. People have an image of you which is very different from the person you actually are. What you'll be doing when you come out is not, as some would say, destroying the person they know but really just showing them who you really are and how their view of you isn't accurate. Make sure that you treat it kindly though, don't get defensive, and really understand that, to them, you'll be killing off the person that they knew. Eventually they'll realize that is not the case but it'll take time. It will hurt. It will hurt them and it will hurt you. This is a growth spurt that everyone here has gone through who's gone down the road of transition. It's hard and it is not comfortable but, after a while, life will return to normal just, you won't be faking who you are anymore and the stress of doing so will be gone from your life.
That said, hon, I really do understand. My mom went through so much anguish when I came out to her. She blamed herself, god, me, my father. She cried for days and I had to brace myself for the fact that would happen before I opened my mouth to tell her the truth. There's no way to make it all go well at first if its not going to. Maybe you'll get lucky and your parents will be fine with it. Maybe not. Take it slow, know that you're doing this as the best thing for you and know that, ultimately, that's what your parents want.
Make peace with this in any way that you can though as it's coming if you're serious about this and putting it off once you've made up your mind is just going to do you more harm and isn't going to do your parents any good.
We'll be here to talk to when you need to deal with what happens. I'm hoping that there won't be much to deal with.