Is it lunch? Supper? A dinner party?
Like Luckygirl and no doubt many here, since being widowed I have found it hard sometimes to “reach out” and risk rejection. When you are alone/lonely you can feel more sensitive and it is harder to bounce back.
There is a fear of appearing needy and it is hard not to feel rejected, even if there is a perfectly valid reason for being turned down.
You have two options
1) think up an excuse (positive covid test? Bad cough? A domestic crisis?) but be prepared to feel bad about it
2) grit your teeth and go. Who knows you may enjoy it, bring her some pleasure and what is a couple of hours out of your “time poor” (lucky you) life.
Of course you could have obscure dietary needs, so maybe meeting for coffee might be better (see below)
In future you need a better strategy - if unsure about an invitation, “that sounds lovely, thank you, but I do not have my diary with , I need to check the calendar when I get home, etc etc maybe looking at next month? “ and hope your vagueness lets you off the hook. Or use the dietary excuse to dial it down to a coffee out somewhere.
You say you felt it would be unkind to decline as she is lonely.
#doasyouwouldbedoneby