Married 60 years and fed up

My husband was in the Navy so used to being away from home and doing his own thing whilst I stayed at home and brought up our 2 daughters.
He moved into the spare room when I was menopausal and because he couldn’t stand the hot flushes I got every night.
I discovered he was having an affair with my best friend when I came out of hospital after having pancreatic surgery but I was too weak and ill to tell him I knew
We moved to an apartment for the over 60’s when his affair ended since when we’ve lived life amicably and peacefully. We joined an exercise group for older people to help us keep physically fit and up until now have enjoyed a peaceful life but he has struck up a friendship with one of the instructors in our walking group and all my old feelings of helplessness have returned and I’m feeling resentful and unhappy. I’ve never told my daughters about their fathers ways because they think he is wonderful - he was always the magical daddy who came home on leave laden with gifts for very short periods of time and I’ve always been afraid that they will take his side and I might never see them again because I love them and my grandchildren dearly. He can be malicious and is an accomplished liar and my girls live hundreds of miles away anyway. Should I break the habit of a lifetime now that he seems to be coming involved with yet another woman?