But whatever might be the particulars of their separation, her sister's affliction was indubitable; and she thought with the tenderest compassion of that violent sorrow which Marianne was in all probability not merely giving way to as a relief, but feeding and encouraging as a duty. (Ch. 15)

I agree Marianne is quite young. She is still very much under the influence of her mother. Her strong sensibilities are encouraged by Mrs. Dashwood per Ch. 1. I think Marianne’s reaction to Willoughby’s leaving was over-the-top but based on genuine feelings arising from that moment and then fed by her. I thought at first perhaps (maybe hoped too) Marianne’s overreaction had been augmented by the misfortunes and disappointments of the past two years—Willoughby’s departure being the straw that broke the camels back because he meant so much to her. However I think Marianne and Mrs. Dashwood’s over-reaction to parting from each other for what is “expected by all to comprise at least five or six weeks” (Ch. 32) to town weakens that possibility greatly.

Her [Marianne’s] unwillingness to quit her mother was her only restorative to calmness; and at the moment of parting her grief on that score was excessive. Her mother's affliction was hardly less, and Elinor was the only one of the three, who seemed to consider the separation as anything short of eternal. (Ch. 25)