I read what you wrote about love - and I agree. I do feel however that when anyone accepts and reciprocates the love of another person it entails certain moral responsibilities to that person in how we treat them and go on treating them. [in an adult relationship, I am not talking about our children here, which is something else]
The modern idea is that no-one 'should' stay in a relationship which no longer meets their needs. And of course we do not want someone in our lives who doesn't want to be there. BUT and it is a big but, for a long time they accepted and gave back the love we gave them.
We cannot stop loving - it isn't a tap that we can switch on and off, is is like a flow of water or an electrical current that keeps flowing.
What our spouses feel for the new person may or may not be love, only they can know that, but it doesn't give them any right at all to treat us with disrespect, and to fail to give us what is our due as their life partner of long standing. We would expect better treatment from our business partner if we decided to go separate ways.
The pathetic whine from the WAS that they are seeking happiness cuts no ice at all with me any more. Maybe they are, but grown ups still need to behave like grown ups and deal nicely with those they are dumping!!
