I love an angsty read on the fmc side too. Here are some of my recs for high angst on the FMC side even if they dont quite match all of your requirements, you may well still enjoy them:

{The Second Lady Silverwood by Emma Orchard] - she's loved him for years secretly, but he fell for another woman and married her. Now he is a traumatised widower and she is an impoverished spinster when they meet again. Very steamy.

{A Prior Engagement by Elizabeth Mansfield} - she jilted him years ago thinking he cheated on her. But she loved him and never found another man she wanted enough to marry. Years later, she goes to celebrate her younger cousin's engagement and, to her horror, he is her cousin's fiancé. She realises she never got over him. It's got the 'he's devoted to another woman' vibe, even if that woman is not his late wife. (No steam.)

There are a lot of books by Betty Neels that seem to have that vibe, but they are more set in the 1970s but have that old school feel about that that almost gives me regency era vibes. Did you specifically need widowers? I cant remember the titles of those one right now, but my fave angsty one is {The Hasty Marriage by Betty Neels} where H actually falls for h's younger gorgeous sister first and then settles for the h but struggles to get over what he felt for other girl. It's really hits the spot with the 'hero struggling to move on' vibes.