After Willougnhby leaves, Marianne is really following her script again.

At the end of Ch. 15 is this:

They saw nothing of Marianne till dinner time, when she entered the room and took her place at the table without saying a word. Her eyes were red and swollen; and it seemed as if her tears were even then restrained with difficulty. She avoided the looks of them all, could neither eat nor speak, and after some time, on her mother's silently pressing her hand with tender compassion, her small degree of fortitude was quite overcome -- she burst into tears and left the room.

At the start of Ch. 16, we read

Marianne would have thought herself very inexcusable had she been able to sleep at all the first night after parting from Willoughby. She would have been ashamed to look her family in the face the next morning, had she not risen from her bed in more need of repose than when she lay down in it. But the feelings which made such composure a disgrace, left her in no danger of incurring it. She was awake the whole night, and she wept the greatest part of it. She got up with an headache, was unable to talk, and unwilling to take any nourishment; giving pain every moment to her mother and sisters, and forbidding all attempt at consolation from either. Her sensibility was potent enough!

Here are Marianne's rules for grieving:
1. Do not sleep
2. Do not eat
3. Do not speak to anyone
4. Weep constantly
5. Do everything you can to keep reminding yourself that you are sad.

This is not to say that Marianne is not genuinely upset, but the fact that she would find it 'inexcusable' or be 'ashamed' to behave in any other way shows that it is part of her internal script she follows for how to display her deep, deep feelings.

It's striking how similar this is to the behaviour Marianne demonstrated when her father died and she "resolved against ever admitting consolation in future". Also this time she "wandered about the village of Allenham, indulging the recollection of past enjoyment"
and
"played over every favourite song that she had been used to play to Willoughby, every air in which their voices had been oftenest joined, and sat at the instrument gazing on every line of music that he had written out for her, till her heart was so heavy that no farther sadness could be gained; and this nourishment of grief was every day applied. She spent whole hours at the pianoforte alternately singing and crying."