this is going to sound funny but whatever...
do you have a slight bend in your D? a lot of people aren't totally straight down da shaft if you follow.
anyway if you're someone who has a decent bend in your business, maybe that could do it? the lube suggestion is good too, but also just try changing positions.
for example if you're doing it straight on (missionary) and it's on her right side, maybe have her lay on that side and try doing it that way, or laying next to/behind her also. just switch the positioning up when she starts getting sore, that should help