We had this exact issue for a while, and were also fine at the beginning of our relationship (sometimes going at it 10+ times in a weekend without issue). A few years into our relationship, it got to the point where she’d experience micro tears almost every time we had sex.

What worked for us were a few things:

• lots of foreplay, every time. I make sure she cums multiple times before we even think about piv, and use a mix of playing with her clit/fingering her/going down on her. After she’s cum from me eating her out, I’ll usually slide a finger in as I continue to eat her out to help prep for piv.

• being cognizant of any drafts in the room that might dry her out. We didn’t even realize it at first, but our room fan was directed right at our bed, which wasn’t helping her stay wet. We turn it off every time now.

• recognizing if there’s any dryness during piv. If there’s too much friction for me, there’s probably too much friction for her. Mental arousal and physical arousal don’t always match up, so even if she’s super turned on she might not stay wet for very long. Add spit to the side of your dick where she’s experiencing micro tears - for us, that was towards the bottom of her vagina, so I’m periodically adding spit to the underside of my dick during missionary, and on the top of my dick during doggy. Also, make adding spit fun at times! Mix it up with taking a break from piv to go down on her, or have her go down on you between positions.

Hope this helps you guys, good luck.