going to dentist... help?!

i have to get a few cavities filled but the thing is i have a bad gag reflex. i can hardly even do the xrays. another thing is im terrified of being in there. (im 17 by the way). i went today to get a couple on the same side on the bottom. the stuck this thing to hold my mouth open which was very uncomfortable and also put plastic paper thing over my tooth. i got to freaked out and had to stop and do reschedule. is all that normal for them to do? is there other ways they can do it or what?? how can i not be so terrified?

thanks