"O, how fallen! how changed from him who in the happy realms of light, clothed with transcendent brightness, didst outshine Myriads" Beelzebub was one of the brightest angels. He was no longer since he fell into the flames.
"That with sad overthrown and foul defeat hath lost us Heaven, and all this mighty host" Satan is feeling deep despair becasue he jsut lost the war of God. Beelzebub says that he regrets it.
"I regret this."
"I cannot believe I just lost to God."
"Which ofttimes may succeed so as perhaps shall grieve him, if I fail not, and disturb his inmost counsels from their destined aim." Satan wants revenge and wants to get back at God.