The Duchess with an Empty Soul
http://www.mangago.me/read-manga/the_duchess_with_an_empty_soul/
Husband used her magic abilities for himself and cheated on her with her cousin, entered a contract that turned into love with the Duke.
The Reason Raeliana ended up at the Duke's Mansion
http://www.mangago.me/read-manga/the_reason_why_raeliana_ended_up_at_the_duke_s_mansion/
Fiance was going to poison her, she went back in time and sought out the ML for help
They're both historical/nobility if you're fine with that (▰˘◡˘▰)
Remarried Empress, the chapters out is still a few tho
Behind the desk
are there any mangas where the mc is in a toxic relationship and the ml comes in and saves them by showing them what real love is and etc??