SEX help

Hi so I feel like I have trouble having sex. I'm a virgin but I've used dildos, vibrators, you name it. All I can shove into my vagina. But when it comes time for my boyfriend and I to do it doggy style, we can never get it right. He is taller than me by like 7 inches or so. We have done it missionary but that seems like the only position that we can do comfortably. IDEAS? HELP? PLEASE