ML is a regressor, FL married the guy she thinks is her "best choice" and ends up so miserable that she kills herself and leaves a will telling the guy to never remarry because no one deserves to suffer him and his household.

So ML must stop her from marrying this dude again. He initial doesn't plan to marry her himself because to turn back time he essentially had to sign his life away to go back in time and get a year or two to try and save her. However after going through the list of all the men she could marry all were either to young or old, had a bad history with women, had the status to reject her for being an illegitimate child, or would put her in danger by using her ties to the royal family for their own political gain.

Causing him to realize she only picked the guy she did in the first life because she had absolutely no other options. And the only reason he knows it's a bad choice is because of something he could never explain to her, regression.

So his only real choice is to get her to marry him and put failsafes in place to keep her safe after he dies, but he has a limited time to do all this.

The "affair" with the other princess was orchestrated by the queen and he uses it to his advantage to ensure that no matter which princess he wants to marry the royal family will not interfere, which means they can't force the FL to say no and approve the marriage. The rumors also put the FL at an advantage in high society because he is elevated to the same importance as a proper princess instead of just an illegitimate one.

He literally is going to due twice for her, he is doing the best he can given the context the story provides, he thinks he's unworthy of her and self depreciates all the time. He thinks if he dies she won't care and only cares that she is happy even after he dies even if she decides to remarry. The marriage contract and his will are written so that she gets all benefits and no downsides. He is a decent insecure man with only one option given to him. Please do not take the 1 star review at face value they did not get very far into the story.

that it's implied he's a regressor who fell in love with MC, who was trapped in a loveless marriage before he regressed

While he was trying to save a suicidal person, he tells her to fall in love with him while he has 2 years to live because of the artifact he used. What? Do you think she would be fine after you are gone? Go turn back time to have the same result?