When you discover someone has been lying to you, you start to treat everything they say as lies.
Maybe there are examples of outright lies by the mainstream media, but I can't think of them right off. Remember that "lie" means not just that it's false, but that the person saying it knew it to be false. Sure, we've been mistaken about lots of things. Sure, there are biases in terms of what something signifies, what the alternatives were, etc.
I used to believe in 9/11. Then the same people who brought you that story discovered that Iraq had Weapons of Mass Destruction. Anyone who had observed Iraq for a long enough time knew that was false.
But consider the difference. There was a difference of opinion at the time, then after the invasion of Iraq and inspection and the finding there were no such weapons, then lots of soul-searching, egg on the face of neoconservatives, and a new consensus that there had not been any such weapons. It corrected itself.
That gets you to start to treat all their other claims as lies until proven true.
So it's good to be skeptical. But the implication is that you think there is some other source of news that is noble and unbiased. Show me this other source and what it has done to earn credibility. Otherwise skepticism becomes just a license to believe whatever you'd like to believe. There really is objective truth on facts out there, and we can't just give up on trying to find out what it is.