To myself I have to wonder why the expression used is "a sharp sword" what does the word sharp come to add? Many people may look at this question and dismiss it on the grounds that it is an aggadeta, part of a story in the Talmud and not subject to the same rigors of expression as halachah. I disagree with those people on the simple grounds that the same people that so carefully composed, preserved and related those halachot composed, preserved, and related these aggadeta.
In the simplest sense, a sharp sword is more likely to kill cleanly, and requires less effort to do so. So in this context, a sharp sword comes to teach that even if there remains absolutely no window of doubt, no chance of error, STILL know that HaShem can have mercy on you and extricate you from your predicament. This is easier for us to digest in our day and age, we almost take it for granted since movies are full of just such escapes.
This was an absolute chidush in time of David HaMelech and Hizkiyahu HaMelech. Yishayahu just told him: "You have no hope of changing your fate, HaShem has already passed His verdict." Hizkiyahu quotes this teaching throwing it (proverbially) in Yishayahu's face.
It's important for us to remember even until the very last instant.
No negative prophecy is ever final.
The only prophecies that will always be fulfilled, the ones that are non-negotiable, are the positive ones.
Yishayahu knew that, yet he didn't seem to know just how far that went. He thought the negative prophecy doesn't have to come to fruition, but if it begins to, then there is no hope of escape, after all it was foretold in prophecy. That's why he was so sure that he didn't try to help Hizkiyahu HaMelech even after Hizkiyahu requested his daughter's hand in marriage.
It's so easy to think, even when everything is coming crashing down, that it's all over, it's too late.
It's never too late.
The exact quote states, don't prevent yourself from HaShem's Mercy. Never give up hope. Never give up an unwavering faith in HaShem's mercy.
When He took us out of Egypt it was a promise to take us out of every exile, national as well as personal.