I find it interesting how conflicted Tartah is about his situation. He chose to leave with the brims to try and figure out their plan so as to find a way to save Custas, but he never imagined he would have to do it alone, he really thought Coco would go with him,
but of course she didn't, not after he used forbidden magic on her and because siding with the brims is what the brims want. Tartah doesn't trust the brims, but he is fed up with the secrets the wise men of the pointed hats are having everyone uphold, and as much as Coco also wants things to change in terms of the rules of magic and what is and is not forbidden, following the brims is not the way to change anything. She does not want to do things the way the brim hats want because their methods and beliefs are just as cruel as the magic councils.
With everything settling down, Tartah must have had time to think about everything that has happened and see that despite his noble intentions when leaving with the brims,the full extent of his choice may only now be settling in and making him realize the true danger he is in.....he can never go home, he can never see his grandfather or his friends and he broke the trust Coco had for him, he is in a constant state of danger now as the brims will not hesitate to hurt him and Custas if he tries to do anything against them. Even if he does try to go back to Coco, she can never look at him the same way again and that alone will be heartbreaking to Tartah as he realizes he messed up and now things between them can never go back to the way they were. Bottom line, if Tartah wants any chance at repairing his friendship with Coco, he's gonna need to do a lot more than apologize, he's going to need to work to earn Coco's trust again.
The way he kept saying "we", only for him to realize there was no "us" anymore.