The speaker is looking back on a moment when he ended a relationship in the worst possible way. Instead of parting gently or wishing the other person well, he turned cruel. He says he attacked her heart and mind, cursing the idea that she might ever be happy with someone else. His jealousy and bitterness made him “inhuman,” and he left her devastated while he himself felt like death.
Now he sees that his anger poisoned him as much as it hurt her. The memory feels shockingly recent, and all he can do is repeat the word “forgive” over and over. The mood is heavy with regret and self-disgust, but also desperate—the speaker knows he caused real harm and can only beg for a mercy he is not sure he deserves.