The word we translate separate signifieth to divide, soften, or make slippery, which hath made interpreters vary in the exposition of it. But the general use of it, especially in Pihel, (the conjugation in which it is here used,) being to signify a dividing or separating, and the latter signification being secondary, it seemeth most reasonably here translated to separate, or to withdraw. Jeremiah had no further revelation from God which he was under an obligation to communicate; and knowing the city would suddenly be taken, and that he could be no further useful to the people, taking advantage of the withdrawing of the Chaldean army, resolves to provide for himself, designing to go to his own country, to Anathoth, which was in the land of Benjamin; and because he was a noted person, who might probably be stopped (as he was) if known, he attempts to slip out in the crowd of people that were going out. This seemeth to me the most probable sense.
Related Commentaries of Jeremiah 37:12
Jeremiah 37:12
12 Then Jeremiah went forth out of Jerusalem to go into the land of Benjamin, to separated himself thence in the midst of the people.