Albert Barnes Commentary Psalms 13:4

"Lest mine enemy say, I have prevailed against him; [Lest] mine adversaries rejoice when I am moved." — Psalms 13:4 (ASV)

Lest my enemy say, I have prevailed against him - I have overpowered him; I have conquered him. That is, to triumph over him as having obtained a complete victory.

And those that trouble me - Hebrew, “My adversaries.” The reference here is the same as in the previous part of the verse. It is to the enemies that seemed almost to have triumphed over him already, and under whose power he was ready to sink. Rejoice. Exult; triumph.

When I am moved - Moved from my steadfastness or firmness; when I am overcome. Until now, he had been able to hold out against them; now he began to despair, and to fear that they would accomplish their object by overcoming and subduing him. His ground of apprehension and of appeal was that, by his being vanquished, the cause in which he was engaged would suffer, and that the enemies of religion would triumph.