Don't forget that Ross only began his "We were on a break!" mantra AFTER The One With The Morning After which results in Rachel definitively breaking up in light of learning Ross slept with Chloe. The next episode he starts his mantra which just brings back all the red flag behavior of the season 3 episodes that led to Rachel wanting the break in the first place. He doesn't want to feel guilty anymore and really doesn't want to deal with Rachel's continued anger so he decides that he did nothing wrong and she's being unreasonable. Never mind that he spent the entire day running around convincing everyone to keep quiet about him sleeping with Chloe because HE KNEW it would upset Rachel. If he always thought he'd done nothing wrong he'd have said that and stuck with it the whole time but he only settled on that after the breakup. That whole episode pisses me off actually because Ross is being an asshole the entire time: he decides he's not at fault for Rachel's hurt feelings and anger and she should just get over it, he tries to convince their friends to cancel the weekend plans they made with Rachel because he thinks they should prioritize him, and he tells Carol that Rachel dumped him for Mark and makes himself seem to be innocent in the breakup. Then he gets rewarded for his behavior when their friends convince Rachel to be cordial and be around Ross so their friends don't have to choose.
To be fair to the show in season 3 they did an excellent job of laying the groundwork for Rachel to ask for the break, and showed some awareness of how shitty Ross was being about the breakup in the second half of the season but all that went away as the series went on. Even in the series finale Ross references "we were on a break!" in the moment he's getting back together with Rachel which tells me he learned exactly nothing in the seven years since the breakup. TV is full of couples who the audience 100% knows will breakup offscreen after the show ends and Ross and Rachel are at the top of the list.