‘But I call God for a witness on my soul, that to spare you I forbore coming to Corinth.'
So why then had Paul failed in his promise to come to Corinth? He calls on God to witness to the truth of what he says. It was in order to spare them what would have resulted from his arrival had he come in person. He had felt that the result he desired was better achieved by his severe letter (2 Corinthians 2:1-4) and the arrival of Titus among them.
That he felt it necessary to make such an oath shows how difficult the position was. He clearly felt that it overrode the Lord's teaching that oaths should be avoided in normal relationships. Here it was necessary because it was important for the sake of the Gospel to establish the facts without doubt. He wanted them to know that there really was no other reason for his absence than that he had wanted to spare them sorrow.
‘On my soul' probably simply means ‘on me', that is, ‘on what I speak from my inner heart'. Although some see it as indicating something stronger, ‘on my very life'.
In other words he did not want them to be left with the impression that that the reason that he had not come was because he was sulking, or because he was so angry that he wanted nothing to do with them. And a mild explanation at this point might have left them with just such a feeling, and with the idea that his explanation was just an excuse and that he was just being devious. So he was concerned that they did recognise that he was being honest and that that was the true reason, so he confirmed it by this mild oath.
But what does he mean by ‘spare you'? The probability is that he had recognised that he might have to speak very severely about the person in question, and those who were supporting him, in the presence of the whole church, which might have left a longstanding sense of grievance among them. Some might even have been brought in to the situation who were not really to blame, and who might well have been caught in the cross fire, leaving a further trail of resentment. Much misunderstanding might have arisen. This would then have been a hindrance to his future ministry among them. On the other hand his view had been that an Apostolic letter, and a visit by Titus who was clearly not directly involved, would not be taken so personally, and would hopefully strike at the right targets, leaving the way open for a further visit by him.
(That we do not have more details is annoying for the commentator, but it is actually for the good of the church due to the thousands of church situations to which it can be applied, thus giving church leaders an example of unselfish pastoring to go by and to imitate).