HaBO: Historical with Older, Widowed Main Characters

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 could help each other and that he could provide better support for her and her family.

The heroine accepts partly because her daughters encourage/pressure her to accept, telling her that he would be able to support them.

I vaguely remember that one of her daughters may have been pregnant, but I’m not completely sure.

The hero also has children of his own.

One VERY specific detail I remember is that the heroine’s late husband may have left the hero a letter. In the letter, he basically admits that he knew the hero was in love with his wife, but that he was too selfish to give her up.

I believe this is the first book in a very long series. The later books are about their children, and I think the series eventually reaches their grandchildren as well.

I have a vague memory that the cover showed a woman wearing a grey/gray dress, but I’m not certain about this.

I know some of these details might be slightly wrong, but the older widowed protagonists, the hero being in love with her while she was married to his friend, the marriage proposal, and especially the letter from the deceased husband are the things I remember most strongly.

I know many of you will be interested in this one!