The Telos of Dasein

It seems to me that, should one divide the extant “torso” of SZ between the contents corresponding, respectively, to the two nouns of which its title consists, the “waist” would be found precisely between sections 64 and 65.
Yet before (or alongside with) going through this time-part, we need a clear idea of how exactly H. construes temporality per se, prior to its functioning as an "origin" of world and--via it--ordinary time.
This is what I am currently stuck with.
Here, BTW, the analogy of the swimmer properly belongs.
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“To be” one’s possibility (in this case, a swimmer) is not the same as “to be (doing)” anything actually (in this case, swimming). Just as the meaning of "can" is different in, respectively, (1) "I can (at any time) swim (or not swim)" and (2) "I can be a swimmer (because I am one already)".

Can you please explain the meaning of sentence 2 and then contrast it to 1?
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A key phrase is on H. 191:

Das Dasein hat sich in seinem Sein je schon zusammengestellt mit einer Möglichkeit seiner selbst.

This is precisely the Möglichkeit of which (2) speaks. It differs from the possibility for me to swim in that it is constitues the being of my self.
The possibility fo me to swim does not constitute my self; it is my possibility, but not (a possibility for me to be) me.
I (=my self) have to exist already, in order for there to be a possibility for me to do this or that (in particular, to swim).
Also, the possibility for me to swim can be inferred from an actuality, namely, that of my present conditions and past experience.
My being a swimmer, on the contrary, cannot, in principle, be inferred from any actual condition or experience of mine; it is a purely projected (i.e., a priori) possibility.
Finally, and most importantly: the possibility for me to swim at a certain given moment, once actualized, ceases to exist; whereas the Möglichkeit of which H. speaks, remains--despite (or, rather, in virtue of) its having been actualized--a possibility.