I agree -- she is not acting a part, these are her real emotions. I think that's what this quote from the beginning of Chapter 16 tells us:

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.