Heroine does not trust new husband due to her parents bad marriage

Heroines parents were estranged because the mother thought the father had cheated, but he hadnt. Heroines trust issues stemmed from this. Think she has arranged marriage. Hero gets wounded and gets letter from former mistress. think her name is lady pamela. heroine and hero fall in love, and he knows she has trust issues. towards end party at house "lady pamela" is there. hero takes her to his study to talk and he tells her he is happy in his marriage (might be holding her hands) . heroine walks in and tells hero how easy it is for innocent moment to look like something different. Hero is happy because his wife now trusts him. Think older book.