The belief of being prudent, and self-denying principally for his advantage, was her chief consolation, under the misery of a parting, a final parting; and every consolation was required, for she had to encounter all the additional pain of opinions, on his side, totally unconvinced and unbending, and of his feeling himself ill-used by so forced a relinquishment. He had left the country in consequence. (Ch. 4)

I think Frederick played a role in his heart being ripped out. Anne thought he might seek her out after he made fortune enough to marry but he did not—this makes me believe that Anne broke the engagement but tried to make him understand she still loved and wanted him:

He must be either indifferent or unwilling. Had he wished ever to see her again, he need not have waited till this time; he would have done what she could not but believe that in his place she should have done long ago, when events had been early giving him the independence which alone had been wanting. (Ch. 7)

I think Frederick decided at that moment it was all or nothing—now or never so to speak which is what he says in his letter, “A word, a look will be enough to decide whether I enter your father's house this evening or never.” How tragic it would have been if Anne had not managed to get through to him the second time around—I completely understand her concern to assure he understood she wanted him to attend the Camden Place card party.