I believe she's lying to herself and to Ben both, but the song also reveals her bitterness and regrets, IMO.
The lines "every morning--don't faint--I tend the flowers, can you believe it every weekend I paint for umpteen hours?" are so revealing. I feel Sally can't believe it herself that this is what her life has become about, the fact she says "umpteen hours" implies that this life seems sort of interminable and monotonous.
I think the lines "and yes, I miss a lot living like a shut-in; no, I haven't got cooks and cars and diamons; yes, my clothes are not Paris fashion but..." imply that she's quite envious of Phyllis and that she does miss it a lot regardless of whatever excuse she wants to make for herself. Those lines could be played with a sense of bitterness.
There's a lot of emotions going on in this song: regret, denial, jealousy, seduction (I feel she's trying to get a reaction from Ben, manipulating him in a way), etc. It's a brilliant song, Victoria Clark nailed it at the Encores! concert.
"Some people can thrive and bloom living life in a living room, that's perfect for some people of one hundred and five. But I at least gotta try, when I think of all the sights that I gotta see, all the places I gotta play, all the things that I gotta be at"