The next verses in this Psalm must have been the low point for the author. For it is not an enemy who is taunting me—that I could bear.
The author means that with an enemy, you know what is happening. You can expect threats, oppression, and fraud. This is also not about an opponent who treats me without respect—for I could hide from him.
The author could run away, or not care about the threats which are being voiced. In both cases he can arm himself. But the one who taunts the author is the one, his equal, his companion, his trusted friend. They were so close together. And now he is one of those enemies, who are getting dragged along in fraud, injustice, and lies. He is one of those described in Psalm 55:3–5 (noise of the enemy, oppression, the wicked)—one of those for whom I would flee with wings of a dove (Psalm 55:6). It is them who are poisoning the city with iniquity (Psalm 55:10–11). The shock concerning this is made clearer in Psalm 55:14.
12 For it is not an enemy who taunts me then I could bear it; it is not an adversary who deals insolently with me then I could hide from him.