Uncomfortable Sex

So my Fiancé and I have been having sex for about 3 years now, but I yet to find it comfortable to me. Whenever we try to have sex I am in pain. The pain feels like his penis is too big and it is stretching my vagina out.I have tried to tough it out and see if I just needed to get through it once and let it stretch, but that didn't work. He is in the military so we will go long periods of time without having sex. Does anyone understand why I still feel like this?