The Razor's Edge; by W. Somerset Maugham.

The characters are as unforgettable as the story. Where does one even begin to describe it?

The boy who came back from the war changed, the girl who was waiting for him and wanted to have him for a husband and lover - but also a life she could trust, and so married someone else, the journey of the man she loved from her own social set to far beyond where she could not reach him - though it took more than his lifestyle and his decision of marrying an ex-schoolmate who was wasting herself - that she thought she could change, and really had never let go of him - to make her come to realise she could not continue, that she did not possess him and could not even see him any more as a social acquaintance much less a friend.

One would feel sorry for her had it not been for the breaking up of his marriage she managed before he could marry the woman she disapproved of.

Of his journey, one had best read in the original. I have wondered who the main - male - character is based on, for the description of the part in India could not possibly be imaginary. But then again there is no secret or shortage of people with realisation when it comes to India.