I have to take a colonoscopy,is it going to hurt will I be ok?

And why does the doctor have to go through my but,I told him to get a longer tube and go through my mouth to get to the whatever in my he is looking for he just seemed to be annoyed but I'm scared I tried 1time and I felt the veins popping out my forehead and I screamed for my mother,so I don't like people to go back there.Why does he have to go through my I'm scared.