Hi @mjk20 and welcome to the community, though sorry to hear about your diagnosis. Your position is not at all unusual and often people need to work out their own emotions before they can share. Sometimes too people feel they might share with someone close but do not want things to go any further, sometimes worried about what others reactions might be.
Well done for coming here though, everyone on this site knows how hard things can be.
I wonder perhaps though how your relationship might have already changed and if your girlfriend could be wondering what is different. Perhaps it might help you to look at our pages on talking about cancer and see if there is anything there that might help you start the conversation. I am sure there is a lot of things that are perhaps unknown at the moment and hopefully you can approach this together.
<<hugs>>
Steve
