Satan is saying that he was foolish to think he could defeat the all powerful. By definition you could not defeat someone who is omnipotent, because their power has no equal. God created him and knows all, so it is obvious that he can not be defeated. Satan wishes that his pride had not blinded him before he started this rebellion. 

This is an example of the epic boast which is an important literary device. Some of the best examples of the epic boast can be found in Homer's Iliad and Odyssey. Satan boasted he could defeat God and this is what most epic heroes do before they go to war. It makes sense that he does this, because if he did not believe that he could defeat God than he would be senseless for starting the war. He is the leader of an army, and it is his job to instill confidence in his troops. In a way, this makes Satan look good, because if he can make people believe that God can be defeated he must be a great leader.