The whole point of Husbands of River Song is the Doctor finally, finally being able to be on equal footing with River, realising how unbalanced their relationship has been, realising how much holding off because of grief (& potential grief) harms them.
"When you love the Doctor, it's like loving the stars themselves. You don't expect a sunset to admire you back" is the climax of the episode, is 12 realising that the next 24 years are gonna be dedicated to showing River just how admired she is, how loved. That for the whole time he was 11, River thought he was a monolith that she worshipped, because 11 was running so hard from the knowledge of her death (which we see in the minisodes/Husbands too where he is constantly avoiding Darillium because he knows it's her penultimate page (with him)), running so hard from how he'd have to grieve after their last meeting (and like... how would you know when to start mourning someone you've already watched die but keep meeting?).
11/River are out of step a lot of the time, in timelines and in their feelings, even in Angels when they're basically linear, we get "Never let him see the damage. And never, ever let him see you age. He doesn't like endings", River is still in this place where she sees (rightfully) that he's not happy with their relationship because of the grief it causes them both, which is then exactly what he has to tackle in Husbands. That they can be happy together in the time they have if he just lets go of his (arguably premature) grief enough to lean into just loving.
"Happy ever after doesn't mean forever. It just means time. A little time. But that's not the sort of thing you could ever understand, is it?" So the Doctor makes it his absolute mission to understand, to learn, to love her to the point where in the library when River describes 12, he's HER DOCTOR, the way she talks about him you can TELL she knows she's loved, she knows they're good together and comfortable. You don't expect the sunset to admire you back but it can sure as hell choose to.
