Difficulty getting wet during sex

So I 19F am with my 20M boyfriend and we always try to have sex. I am literally always super wet when we’re doing foreplay or just making out prior to sex but then when it’s time for the actual penetration to happen I dry up and it hurts. I’m not sure why this is happening because I am turned on and well prepared but my body seems to panic whenever the actual penetration is going to start. Any tips ?