...by the similarity with the passages you quote to the way Marianne and Mrs. Dashwood mourning Mr. Dashwood was described back in Ch. 1
"They encouraged each other now in the violence of their affliction. The agony of grief which overpowered them at first, was voluntarily renewed, was sought for, was created again and again. They gave themselves up wholly to their sorrow, seeking increase of wretchedness in every reflection that could afford it, and resolved against ever admitting consolation in future."
On the one hand, having lost a father only eight months earlier, it might make the grief over losing Willoughby that much harder to bear. But on the other hand it seems wrong to me, somehow, to express her grief to the same extent she grieved for her deceased father over a man she has only known for six weeks.