Originally Mairon was fighting back with words a bit more.  More thoughts.From Mairon’s P

Originally Mairon was fighting back with words a bit more. 

More thoughts.

From Mairon’s POV, he has lost power here because:
1.) if what Manwe was saying was true, there is no sense going to her for help since she’d just be on Manwe’s side
2.) even if what Manwe’s saying was not true, Mairon is still bound by Manwe’s words since Manwe’s the stronger of the two ainur and Mairon is not able to truly believe it as being untrue. (and also the other reason why he would not able to ask for help from the other Valar since they’d all look like enemy to Mairon)

If Manwe was a maia that were weaker than Mairon, he would have able to think much more logically but he’s not able to because he’s opposed by a Valar.  Logically thinking its obviously a bluff but Mairon’s not able to ignore it.

Also, I think Melkor did bind Mairon with words like Manwe here, and took control over his actions and ultimately Mairon’s fate of being Melkor 2.