Can't have sex

I don't know why, but it feels like my body is shutting the D out whenever my boyfriend tries to penetrate me

We have been dating for 2,5 years but we haven't had sex (penetrative) even once 🙈

We usually do foreplay, and I get superwet and horny, but whenever it's time to do stuff, my body just shuts it out

He can't even enter it a little.

I can approximatly insert 5 cm of ONE finger, but not more. We have tried using lube, but it wont help

I know that the first time won't be like in the movies, but the pain I experience is way too extreme for me to even force ir in. What should I do about it? Could it actually be a serious problem, like a disease or something?

Or am I doing something wrong?