If she really thinks it's painful you might want to spend more time warming her up. You've gotta get her just as excited as you about it. Spend more time performing fellatio, etc. You might want to buy a smaller dildo too. If you're 8 inches, 6 around try buying a 7/5 and easing her into it. She has to get used to a big dick and once every 4 months isnt gonna cut it.

also, you gotta talk to her about it. My long time girlfriend used to have a pretty mediocre sex drive. After a few conversations where I expressed to her how important it was to me, how its biological, etc she was much more open and understanding. She was willing to give it up more. The more sex we had the better it got and now she's acting like a full fledged whore in the bedroom (which is fantastic).

Don't give up and good luck.