Going Dry During Sex

When me and my boyfriend have sex, I'm extremely wet in the beginning. We often times change positions several times and I feel myself slowly drying up down there. Eventually, (after I reach the big O myself) I'll start to feel extremelyyyyy dry & lots of pain. His penis struggles going on because I'm so dry. My bf usually has to spit down there to help...and by the end of the sex just hoping for him to just CUM already! Is this normal? What are some tips on fixing this?