Honest question - what part do you not believe?
It's easier for me to say what I do believe.
That Russia is moving troops into Ukraine seems to be well verified by multiple non news sources as well as official sources. I can believe it is true.
Most everything else is subject to fabrication and distortion from the government/media psychological warfare systems we've been manipulated with for decades.
Why, after having been lied to about just about every major event in our lives are we willing to believe the system about anything? I don't have to believe what they are saying is all lies, I just don't have to believe they are telling me the whole truth.
The system has failed us as a truthful source of information. Everything that is official information or is reported by "legitimate" news sources is potentially a flat out lie, carefully curated "truth", purposeful disinformation, manipulation, misdirection, persuasion and most likely has ulterior purposes beyond informing the public. This isn't a conspiracy theory. Any single one of us could start a list of official lies we've been told by government or that has been promulgated by our media and the list would be long. We would then be able to add to that list every day or every so often as we remember another "not truth".
The thing is, it's easier to just go with the flow and not rock the boat. Generally, none of the official nonsense seemed to impact our day to day lives substantially. That wasn't really true though and these last two years have driven that home. Reflecting on this reality of official bad information flooding society has been something I've been thinking about for years now.
When people in my life who I have direct contact with lie to me regularly I stop believing what they tell me. Usually, I stop interacting with them as much as possible and I certainly don't make major life decisions based upon on their testimony.
Our government regularly lies to us, the media regularly lies, our major corporations go along with the lies, the universities and educational systems are all in on the multiple streams of lies.
Why believe any of them? I know it's easier to fit in if you go with the flow but at what cost to your own understanding of reality? They are demonstrable liars, if not liars then, being charitable, they are often wrong in catastrophic, amazingly 180 degree ways.
This mindset is still a work in progress. I still find myself wanting to believe. It would be great if the MAGA/Trump rhetoricians were truthful, they won control and made things better. Unfortunately I've noticed the cracks in the Trump storyline and I can't unsee them, not to mention the cracks in our official mythos. I really want to believe there is a movement that believes in the America myth that can get voted in and "make America great again". I'm just working hard to not ignore the incoherencies in that whole storyline.
Anyway, you asked, I hope that sheds light on where I'm coming from regarding (not) believing the authorities.