IMO, you may need to decide on a reason and definite date - and maybe the reason can give you the date - like you will be away for a time, family needs help, etc, beginning January 1, or something like that. Or you are 'officially' retiring January 1. (You don't have to give them any reason - but if/when they ask if you can stay longer, be ready to say, I'm sorry, but no.)
The company may be a little displeased at having to replace you - but they should ultimately be HAPPY for YOU and whatever your future plans are. It's usually a 'pain' to replace someone who's doing a good job, etc, but it's also a normal process of business - they will survive.
I think you may feel good when you 'honor' and regard yourself in this. It may surprise you.
If you find that you miss the day to day activity, co-worker friendships, etc, you can find a place to volunteer, or join a club, or something.
Good luck.