The problem is this is when we get things like the infamous slap scene. In the situation, which is totally in her character as I've said before many a time
The slap was in character? If it were in character, we wouldn't be seeing so many people complaining about it. I think the slap was antithetical to her character, even considering a mental breakdown (I don't think the breakdown itself was OOC, just The Slap). Claiming that Sonic was being selfish didn't even make the slightest bit of sense (which Sonic called her out on, but that didn't make the fact she did it any more believable), especially considering he was acting the way she'd always acted and the way she'd always want him to act, putting his country's needs above his own. Seriously, where did this idea even come from?
Ugh...Again?
1) She's somewhere between ages 15-17. I don't know how many teenagers you know who could take a rejection like that and not blow it out of proportion, especially in a state of instability that she was clearly in.
2) Sonic just died and came back to life. Based on past events such as her father being nearly killed when attempting to retrieve the Sword of Acorns, and her Mother Being in the Coma, and even having Rotor empty the fuel in the Sea Fox so Tails won't get hurt playing in it, Sally is way overprotective to those she fears to lose. It wouldn't be the first time she overacted to a situation.
3) Settling down and having a family, specifically since Elias at the time gave up trying to be king, would be in the best interest for The Kingdom of Acorn. With the King constantly being either away on business or sick, having someone able to take over at hand would benefit the masses. On top of that, Sonic is a hero that many look up to, so he is an ideal choice for that role, considering Sally would be doing most of the ruling anyways leaving Sonic mostly as a figure head. While yes Sonic's skills are best suited elsewhere, Sally wasn't only thinking of herself during the event.
4) On the same note, Sonic wasn't only thinking of everyone else. On many occasion Sonic has ran away from a romantic or relationship building situation. He's flat out told the reader he's not ready yet before. It wouldn't be out of character for Sally to see him using an excuse to avoid doing something he doesn't want to do.
5) Right before Sonic "died" their marriage and stuff was all set into motion. It was expected until it was wretched away when Sonic sacrificed himself to save the day. Would it not be reasonable for Sally to have wished it to continue where it left off? To suddenly have her desires to be stolen away again, is she only supposed to take that smiling? No. Sally's a rebel, she fights for her dreams. She didn't want to give up without a fight. That is completely in her character.
Honestly we all look at Sally as someone who is Self-Sacrificing, always planning ahead, and near perfect that we forget that is only her outer shell. She's allowed to be selfish, immature, and make the wrong decision and still be in character.