gamma_phi: YoungHorse,
First off, THAT'S REALLY THICK! With that being said, you've got to take things in your own hands, literally.
1. Lots of foreplay. She's gonna need it. She's gonna have to loosen up (again, literally) for you to penetrate her at all.
2. Make sure the tunnel of love is well greased for the onslaught. Using lubes like KY should help ease (sorry for all this pun) the tension.
3. Use your hand, on her. Start by using your finger, then two, then three, and perhaps even four to get her nice and open and able (this should be with #1 but needs to be addressed separately since you're a lot thicker than most). You might even consider using a dildo on her during foreplay to help to open things up a bit.
4. Feel her out. Know when she's ready (or ask). When she's ready, go SLOW! Something I used to do when I was younger was to insert the first inch for a while, working it around a bit. Then two inches, then three and so on until I was fully inserted. I didn't do it because I was BIG, I did it to drive them wild. After teasing her like that for a while, she'll practically be begging for you to stick it in (and a while means something 15-20 minutes.)
Take your time, no need to rush things. Listen for the verbal and nonverbal cues that she'll be giving. That's about all I can do for ya pal. Good luck with that, *ahem*, monster.