I haven't seen this movie but the premise seems a bit overly emotionally dramatic to me.

Even if he still had feelings for Katya that does not negate the 45 years that he has been with Kate and all of her married life was certainly "real"

Perhaps it's me but the sort of "painful questions" posed strike me as being the sort that would be posed by a young immature person not those that would be contemplated by a mature person who had spent the majority of their life with a partner.

I had a bad break up with someone I Ioved deeply when I was about 24. I estimate that it took me about 20 years to fully get over the loss. Despite that, it didn't detract from my love for my wife. It was simply another layer of feeling that diminished and changed over time. There are certain elements of that relationship that I still think about and take pleasure in, but I don't yearn for it to return. We are emotionally complex creatures, and I would be shocked if a woman insisted that I not have any feelings for a past love(r) just as I would expect her to be shocked if I insist that she not have feelings for one of hers. The past can't be changed, and it should not be discarded. It is part of who we are.