My GF is always in pain during sex?

I have been with a woman for 6 years and everything is amazing except in the bedroom. Every time we have sex she tells me she is in pain. We have tried different positions, creams, lubricants, condoms etc... but still she is in pain. It has got to the stage that I don’t want to have sex anymore because I do not get any enjoyment/pleasure when I see the pain in her face, and when we do have sex she just wants it over and done with as quick as possible for obvious reasons. I think we men like to believe we are all bigger than average but I’m sure mine is just about the average size, my fianc is fairly petite which may be a contributing factor and I have a fairly large build. My fianc is about 168cm tall and weighs about 50kg’s and I am 190cm tall and 94kg’s.

We split up for a year because of the sexual problems and a few other issues, and when we were apart I explored what was out there and to be honest it was fantastic because for the first time in years I had awesome amazing sex. I was with woman who craved me and craved the sex which made me feel on top of the world!!! Now we are back together and it has only been 2 months and already the lack of sexual desire from her side is creating issues again. I understand she is in pain hence the reason I do not want to do anything for fear of hurting her. I want to be with her but just doing the deed because that’s what is meant to be done does not do it for me and I believe being sexual with your partner is a very important part of the relationship, and we just aren’t clicking... What can we do to resolve this?