Can't decide!

I tend to get lectures about how I need to make my own decisions, I would if I could though. I can never make up my mind and I have no idea why.
"What do you want for dinner?"
"I don't know."
"Which shirt do you like better?"
"I don't know."
"How's homework going?"
"I don't know."
"Are you doing your homework?"
"Uhhh...yeah? haha."
"What do you want to be when you grow up?"
I don't know yet, I can't decide."
 The list goes on an on, and it doesn't really matter what people ask me I either say "I don't know" or come up with something pointless to say. I really hate how I can't decide and I know my parents don't appreciate it very much either. I really know things, I'm not stupid I think it's just a normal teenage thing. And then there is the things I change my mind about all the time.
"what's your favorite color?"
"purple I think? Wait no! it's orange now!"
"Are you still wanting to be a vet, I know how you like to change what you want to be a lot."
"Yeah, I changed it again, I want to be an Opthamologist."
*Next week that will probably change AGAIN*
But I've now finally realized that I have a couple more years till I have to actually decide. I'm just thinking about my future big time, that's what I do and I'm hard on myself. I don't mean to be, but I guess that's how I've been getting my 4.0 GPA and already doing stuff to get scholarships. I'm on my way to a great future, I hope. I want to learn to make decisions right away. I need to make my mind up because one day I could make a stupid choice and my world could be turned up side down. I don't want my life to be full of "I don't know". I'll make my mind up sooner or later.