I don't think it's abnormal emotional intensity, just a bad behavior pattern; lots of children do this at some point in my experience. What you need to do is treat it as a training problem. When you drop her off, if she's fussing, say goodbye calmly and just turn around and walk off-- don't say anything else, don't have a sympathetic look, don't turn around or linger, or any of it. On the better mornings, reward her with an extra hug. The behavior will eventually vanish, because there won't be any percentage in it.
Yes... I second this. My daughter was like yours, and it carried on well beyond 3 because I fed into it. She was so intense and traumatized by my leaving (she's NOT social, like your daughter) that even the experienced preschool teachers were not sure how to handle her.
Anyway, to make loooooong long (lol) story short, her grade 2 teacher saved us both by putting into action what Iucounu suggests. At line up she'd calmly insert herself between DD and I and quickly pull DD into the school. (The first day of grade 2 she actually had to carry her in, kicking and sobbing while the other kids curiously looked on). By the end of the year she was totally fine.
