The long and the short of the matter___________that we cannot continue like this.

The phrase that best completes the sentence is "is simply this."

So the full sentence would read: "The long and the short of the matter is simply this: that we cannot continue like this."

This expression is often used to summarize a complex situation in a straightforward manner, indicating that after considering all aspects, the essence of the issue is being distilled into a simple statement. It emphasizes clarity and directness, which are key in communication, especially when discussing challenging circumstances or decisions. The phrase "the long and the short of the matter" suggests that after weighing all the details and nuances of a situation, one has reached a clear conclusion.