I believe this is No More Turning a Blind Eye.
Ophelia Heavenwalker dies a pitiful death, succumbing to a mysterious illness at the exact moment she witnesses her husband’s infidelity. So when fate gives her a chance to redo the last three years of her life, she decides she’s done being a doormat. She’s taking back her name, taking back her estate, and taking back her life! Now’s the time for Ophelia to enjoy her independence and budding romance, but will that be possible with her failing health and spiteful ex-husband still in the picture?
The protagonist, Ophelia Heavenwalker, was the sole heir of a wealthy count. When she met Stein at age 19, she fell in love with him at first sight, and, before long, they were married, against her father's wishes.
However, Stein only married her for her money, and had a mistress, who moved in them with under the pretense of being his student. At age 25, Ophelia died from an unknown disease, which she speculated might've been a curse from Stein. The last thing she saw before dying was Stein and his mistress making love.
Then she awoke in the body of her 22-year-old self, and recalling the last three years of her previous life, resolved not to repeat her mistakes, and to pay Stein and his mistress back for their cruelty. Soon after, she met with an attorney to initiate divorce proceedings.