Welcome to the forums– glad you decided to come here and share. The ending of a long relationship is always devastating, and being the one to initiate that has its own set of challenges. I can tell from your very eloquent post the consternation and pain you're dealing with, and how hard the past year has been. One thing that doesn't come through to me, though, is uncertainty.
It sounds like you both have tried hard to make this work, and that although you have anxiety around the conversation itself, you have decided for yourself that this is the right move. Start with that and hold on to that. Perhaps this is even a conversation she herself has thought about, and similarly struggled to get the words out. Unless you have good reason to be concerned for her mental health, I think it is sensible to steel yourself and just start a fairly casual conversation that eventually leads to the topic of your relationship.
It will be painful and uncomfortable, but if what you say is true about this long-lasting relationship just not seeming to click anymore, there's no need to feel guilty. As long as it is done in good faith, this is a selfless thing to do for your relationship, especially if you want to continue to be in one another's lives going forward. That said, I also think that if you have been together for so long and she has few friends and family, you may be called upon to be part of her support network– obviously less than ideal, but I think a fair obligation to someone you have shared your life with for so long.
My thoughts are with you during this tough time Chopper2020, and I hope you'll come by again to keep us updated.