Too tight

I have been with my boyfriend for 7 and half years. We have sex often and each time it feels like I am having sex for the first time. His penis is big but my vagina is also really tight and it hurts. The pain goes away after a few minutes but after we are done I am usually sore. I don't understand how after so long it has stayed so tight and I don't know what to do about it. Any advice?