HaBO: Historical with Older, Widowed Main Characters

Help A Bitch Out This HaBO is from Anna, who wants to track down this historical romance: I’m trying to find a historical romance, but I don’t remember the title or author. What I remember: The heroine and hero are both widowed and older, probably in their 40s/50s. They both have children, and most of their children are already adults. It takes place in England, and they are aristocrats. I think the hero is a Marquess, but I’m not 100% sure. The heroine’s late husband was a very close friend of the hero. They were friends and remained friends even while both were married. The hero had fallen in love with the heroine many years earlier, while they were both still married, but he never acted on it because she was his friend’s wife. The hero did not have a good relationship with his own wife. I remember his wife being a particularly unpleasant person, and I think this was one of the reasons he fell in love with the heroine. His wife died before the heroine’s husband. After becoming a widower, the hero continued visiting the heroine and remained close to her. After the heroine’s husband died, the hero eventually proposed marriage to her. His reasoning was partly that the two of them co...