When evil-doers came upon me,
To eat up my flesh,
Even my adversaries and my foes,
They stumbled and fell.
He casts his mind back to the past, and remembers how his enemies had tried to destroy him But no matter who they had been, whether internal enemies or external, they had all stumbled and fallen. None had been able to prevail against him. They had been unable to ‘eat his flesh', that is, to destroy him. And the same was still true. The Hebrew ‘past tense' reflects not so much the past, but the sense of definiteness.
Significantly when the greater David came His enemies would be allowed to ‘eat His flesh' (John 6:53) by destroying Him. For it was only through doing that that light (John 8:12) and salvation (Matthew 1:21) could be made available to His people as they too could ‘eat His flesh' by trusting in Him (John 6:35)