A.T. Robertson Commentary 2 Corinthians 12:19

"Ye think all this time that we are excusing ourselves unto you. In the sight of God speak we in Christ. But all things, beloved, [are] for your edifying." — 2 Corinthians 12:19 (ASV)

Ye think all this time (παλα δοκειτε). Progressive present indicative, "for a long time ye have been thinking."

We are excusing ourselves (απολογουμεθα). He is not just apologizing, but is in deadly earnest, as they will find out when he comes.