On my best days, it's questions like this that really make me love being a historian, and that make this job properly exciting. On my worst days I'm as despondent as you - how can we really know anything about the past? What if it's all just made up bollocks? Thankfully I have more good days than bad days!

The reality is somewhere in between, I think. What I'm going to say is very much influenced by my work on Roman history - the situation may be different for other periods and contexts, although I expect the skills and general assumptions are the same.

We can never trust our sources to be completely impartial - they're all written by human beings, with ideas and opinions and biases, whether intentional or not, and whether they declare them or not. As you say in your question, this is true of current affairs reporting too, and there are plenty of recent examples of politicians, journalists and other prominent voices (name no names, of course) skewing stories and evidence to suit a particular agenda. The core skill of a historian, at the absolute bedrock of what we do, is the ability to analyse our evidence carefully and critically. We have to go in with our eyes open, alive to the potential influences that might have been acting on those who wrote the texts we read, and the messages they were trying to communicate to their intended audience. We don't consider these things in isolation either - if there are other sources written at the same time, or about the same topic, we check the two (or more, if we're lucky) accounts against each other and see if they match up. If they don't, then we try to think about the possible reasons why two writers might tell different versions of the same story. Are they pushing particular agendas? Are they emphasising different themes or aspects of event? Are they writing for different audiences or purposes? This is an ongoing process, and one that will never be finished - we will never be done with studying history, because our understandings are constantly changing. We learn more as we go, we think in different ways about how human society works and how stories get told and retold.

Here is an example to illustrate what I'm saying. Again, this is Roman, and very specific. Nevertheless, it shows the importance of the skills of source criticism for historians.

At the beginning of his Res Gestae (basically his CV) the first Roman emperor Augustus says this:

At the age of nineteen, on my own initiative and at my own expense, I raised an army by means of which I restored liberty to the republic, which had been oppressed by the tyranny of a faction.

If you didn't know the details you might think "fair enough, good for him" but there's an awful lot going on here that needs unpacking. A private citizen raising an army was technically illegal - Octavian (as he was known at this point) had no legal authority to command troops nor to make war, especially not against other Roman citizens. It's even worse when you realise the the tyrannical faction from which he claims to be 'restoring the Republic' is Mark Antony, who was consul (the head of state of the Roman Republic) at the time. So what Octavian was doing could easily be interpreted as an armed insurrection against the Republic, rather than restoring its liberty. However, Octavian went on to win the civil war against Antony, and so it's his interpretation of events that is preserved, but of course we need to be careful with it. Thankfully other Romans wrote about this period - we have Cicero's contemporary speeches, as well as Suetonius, Tacitus and Cassius Dio who all wrote later - so we can cross-reference Octavian/Augustus' words with their accounts. That doesn't mean we can just reject the Res Gestae as totally unreliable - it's still useful to us. It tells us that, even decades after the events, Augustus was seemingly still wary about his reputation, and needed to present his actions in particular ways, not just in Rome but in the provinces, where copies of the text were also displayed on temples.

This is just one line of one text written by one person - historians (good ones, at least) do this kind of analysis with everything that survives from the past. It takes years of careful study and lots of reading to learn how to do it properly, and we still get things wrong. To me, on my good days, this is the absolute joy of history. It's a massive, unsolvable jigsaw puzzle, with no picture on the box and half the pieces missing. There's always more to be said, more to learn, more to argue about. I wouldn't change it for the world.