This person has failed to live up to being the proper lover, but their feelings for the other person haven't changed.

Given the pattern of behavior that you yourself have identified as harmful or hurtful ("infidelity?"), I would say that person has indeed changed their feelings for 'the beloved'. Love could still be there but something else has entered the scene that is not love.

But your premise is essentially unprovable. Love is not a thing, love is everything. All you need is love. Did you miss the message of the 60's?

Love cannot be wrong nor can it do wrong. To put faith in anything less is selling yourself way short.

C'mon Markaba, yer a super smart guy. Even simple sadlife can understand the perfection of love.