I think Marianne believes she acts as the dictates of the Sentimentalism and Romanticism she has imbued from literature and her mother demand. I have never thought she merely wished to look as romantic as possible rather than actually live as a true romantic. If repeatedly feeding her sorrow until she is overwhelmed by it shows that it’s not her feelings that truly matter to Marianne then was that also true when she and Mrs. Dashwood did the same after Henry Dashwood’s death? Neither Marianne or Mrs. Dashwood attempt to moderate, govern or hide their feelings and while voluntarily renewing their sadness may be more understandable after a loved one’s death was it not just as misguided as it is after Willoughby’s departure?

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)

While Willoughby’s temporary departure does not deserve such a violent reaction IMHO I also do not think Marianne is pretending to feel worse than she actually does. Elinor (above) believes Marianne’s affliction was indubitable meaning it “cannot be doubted; patently evident or certain; unquestionable” (dictionary.com) so it appears Elinor feels Marianne’s grief reflects her feelings. I think Marianne’s feelings are an overreaction and how she displays them is as well. Elinor’s thoughts (above) also give an explanation for Marianne’s recanvassing. The characterization of recanvassing (feeding & encouraging) as a duty suggests she feels bound (by her dedication to sentimentalism and romanticism) to first give herself up wholly to her sorrow and then seek wretchedness in every reflection that could afford it. There is some support for such a view in the on-line Literary Encyclopedia:

Sentimentalism entered all literary genres the novel, essay, poetry and drama. But the cult of sensibility was largely defined by fiction from the 1740s to the 1770s. This fiction initially showed people how to behave, how to express themselves in friendship and how to respond decently to life’s experiences. Later, it prided itself more on making its readers weep and in teaching them when and how much to weep.

Sensibility (1740-1800)
http://www.litencyc.com/php/stopics.php?rec=true&UID=1003

The ideas of the French writer Jean-Jacques Rousseau (published 1750-1778) that stress the importance of following nature and human instincts, were developed in England in the cult of sensibility or sensibilité, emphasising feeling in response to contact with people or the natural world…

British Romanticism (1780-1850)
http://www.litencyc.com/php/stopics.php?rec=true&UID=1225