Multi clause sentances
I don't regret having been the jerk that I was.
Used as a Relative Pronoun (to be used with the subject). The noun is "jerk" and the restrictive clause is "that I was", this restricts jerk and explains the specific type of jerk
Note that "that" as a Relative Pronoun is used with the subject, not the object. If the noun was the object then "which" would be used in place of "that".
Was it me, that you said it to?
Used as a Subordinate Conjunction. The main clause is "was it me", the conjunction is "that" and the relative clause is "that you said it to"
It feels like just yesterday that winter break started.
Used as a Subordinate Conjunction but the other way round. The main clause is "winter break started", the conjunction is "that" and the subordinate clause is "It feels like just yesterday"