I guess part of this depends on what kind of stuff you're doing. If you're playing 5 page piano concerto's that would be a bit more difficult than if you're playing 4 chord church hymns. With your years of experience all that music is in you you just don't know it. Set up Biab next to your piano and hit play. Without looking at the music or the screen just start playing along with the song. Who cares at this point if you mess it up, it's just you. Just start doing it and listen, it will come. The other method is how I memorized Fantasy Impromptu. I needed to work on various sections of that piece anyway so I would practice a passage and as I learned it I would stop looking at the music. Since I'm a long time stage performer I'm used to memorizing everything so for that you have start right at the beginning and force yourself to do it. As you know everything is in phrases, just do them one at a time and learn them, that's it.
Here's something else. It doesn't matter of you screw up a passage or not when you're performing alone unless it's a university performance in front of your professor. I completely understand that when you're used to sight reading everything you put tremendous pressure on yourself to do it precisely correct and if you suddenly forget something you can completely lose it and fall apart. Playing at your retirement home is no big deal so just relax, if you mess something up they usually can't tell unless you tell them and stop playing. Just keep playing some pretty chords and pick it up again.
Another thing to help you memorize a song is to start with the melody only. The backing chords flow from that so you can simply noodle the melody with your right hand and just "find" the right chords with your left. When you're playing from the music stop and look "inside" the song from a music theory perspective and see what kind of chord is being used for a particular melody line. I'm assuming you're reading actual piano music, not fake book charts. Music usually doesn't have the chord symbols written above the staff. If it does, that would be great because you can simply learn to play the chord and if you don't play the voicing exactly as written, it will still work. If you want to memorize a song exactly as written fine but for performing at a retirement home, that's really not necessary if you know the melody and the chords. As you get used to this your ears will tell you that yeah, you should play that Cm7 in a different inversion with the melody line but it still won't sound like you don't know the song if you don't do it exactly right. I'm starting to ramble now, but my point here is don't get too hung up on every single note on a piece of piano music. Know the melody and the chords and just put them together by using your ears.
Working with Biab is a tremendous help for this.

Bob

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