The princess really let herself go
Where did they go wrong? Is it because they barely knew each other when they got married?
You answered it yourself. They didn't really have a connection and mostly only did it because that's what was expected. They also didn't seem to really mesh either. They tried to make up for it with enthusiasm early on but thaf failed. Outside of their political duties and that they both love their son they're just kind of breezing by on politeness.
And then probably aided by his time as a brutal wage slave and paper pusher he's done a fairly good job as one of the highest minister's in the nation and absent his relationship with his wife seems to be getting along with his stepfather and king enough that the man is considering making him King instead of just a Prince Consort.
And as someone else pointed out divorcing a princess is hard to say the least lmfao. Neither of them also seem fo really hate each other so they're just coping with the mild unpleasantness.