So... I confess... I've been "shopping" for a good philosophy forum for some time now- and I happened upon this page, and saw the post of confusion about Dasein's- for lack of a better word- "historicity" (it seems Heidegger scholars and translators are really fond of inventing new words)- and thought to myself- hey! Maybe I can help! Or at least direct you to someone who can.
You may very well have it under control- Heidegger is very very hard to read. I happen to be a(n early) Heideggerian, so, I'll share some thoughts.
I'll admit first off that my experience of Heidegger is basically Being and Time plus some snippets of later work- and the Introduction to Metaphysics is not a work with which I am familliar- but, I do know something about the history of Heidegger's career, and about some of the common misconceptions about Heidegger's project- not at all my own insights of course. Someone else can probably do it more efficiently, but, as chance would have it, you've got me.
So, the early Heidegger- circa Being and Time- was reputedly interested in "investigating the meaning of being" - and in particular, that of Dasein- which we are to understand are things like us. Now, this isn't -quite- right, in fact, as my favorite Heidegger scholar Thomas Sheehan has pointed out. What Heidegger is really interested in doing is clarifying what he (either Heidegger or Sheehan's wording) calls 'the clearing of being'- that is, how it is we make things intelligible. So, there is rich discussion in BT about mankind's "discursiveness" (understanding objects as tools and that sort of thing) and "temporality," etc. As of the early 1930s, Heidegger's focus shifted a little bit and he became very interested in history. He also started getting worked up about what he called "the mystery of being." He also got worked up about a few German rabblerousers- the Nazis.
I recommend Heidegger before 1930. But, in both cases, the project was actually out to do metaphysics in a new way- by describing how things in the world become accessible and intelligible to us.
Anyway, Heidegger made sense of the individual as what he called a "thrown project"- that is, sentient, free, etc. but subject to the meanings prescribed by its immediate context and further just plain subject to meaning. But for Dasein to be "authentic" in its being, Heidegger maintained that with the meanings absorbed from one's context, one needed to do something heroic- to put a twist on things and make them one's own. (I realize now that I'm repeating a summary given to me by a professor- secondhand information- if it serves to clarify the point, great- if I'm way off, let me know quick so I can go nag the professor) Further, that, at certain times in Heidegger's career, meant doing what one's own era in history called for. For Heidegger that meant saving Europe from the nihilism he saw it mired in, and, well, joining the Nazi party.
That's somewhat accessory to the point when it comes to Heidegger's basic metaphysics:
Its important to remember- Heidegger is doing hermeneutics. Dasein's being is "in each case" its own- but nevertheless Dasein is not "the master of meaning"- it grasps the objects in encounters by means of meanings that are always a product of being-in-the-world. So, you could characterize the being of Dasein in terms of a mix of activity and passivity- acting but with an understanding passively absorbed from the World (and the "They"- that is, the public sphere). So, your original question- you might be able to see this already- is dead on. The totality of history is how we have to understand Dasein's being- Dasein is "thrown" into a world that's been around for a very, very long time- and its being is wrapped up in that world in all of its qualities.
wow, so much language with so little to say. For actual *clarity* on the subject, I recommend reading Tom Sheehan- in the religion department at Stanford, who has a lot of really clear and interesting articles about Heidegger. Particularly great stuff about Heidegger the man, but, his philosophical renderings are -extremely- helpful:
Tom Sheehan's webpage
He has a large number of articles online. That should help if you run into any more questions.
There. That's all from your friendly wandering Heidegger-dork.