I'm upset that my future FIL, who has dementia, couldn't remember my name
Do you have an issue with your future FIL having dementia? Can you imagine what he is experiencing?
I do know but I can't let this get in the way of our relationship. He still is the greatest man
You should be empathetic of someone's medical conditions. They may not be able to control their actions or thoughts at times. Try to have patience with them and be there to help if need be.
Why would I do that?
The fact that your future father-in-law couldn't remember your name has nothing to do with how much he likes you. His loss of memory is probably very hard for your future spouse to deal with. Don't you think you should just accept him and enjoy your time with him while you can and not make it harder on your fiancee?