I think that the quote really reflects much of the sadness within Winston’s predicament. My most basic read of this quote is that Winston’s moment of renouncing Julia only happens with his fear of rats being exploited. It is at this moment that he is broken, his resistance against the Party exposed, and the reality that guides him is one in being a tool of the establishment. I do think that while his weakness does curse him at the moment of interdiction, I think it’s unrealistic to suggest that Big Brother is not powerful and strong. Given the social, political, and technological means demonstrated in the book, it is nearly impossible to say that it lacks strength. I think that Winston’s weaknesses due hurt him and help to defeat him, but I also believe that it was an uphill battle for him to seek victory against such a mighty adversary that controls everything and uses each moment as a chance to consolidate even more power. It is in such a design that individuals lack power and, weakness or internal strength, this configuration helps to devalue much of the individual hope for progress and redemption.
One of the things that strikes me about Winston and his willingness to pursue his revolutionary activities is the stark difference between he and Julia. His is a calculated approach, one that he hopes will lead to something larger in the form of a movement among the proles. Julia's approach seems to be a devil-may-care, do anything that appears to be an act against the party.
Because of the fact that Winston hopes for something greater, when he begins to see that his work will likely come to nothing, the futility of it begins to destroy him.
So in my mind, you could concentrate on his expressions of doubt, his expressions of hope as well for this uprising from the proles but then the crushing realization that there will be no such thing, these might be good supporting details for your essay.