Early in the pandemic, it was claimed that masks were not required. It was later admitted that this was to help the shortage, at the expense of the truth, and somewhat at the expense of the public [1].
President Biden went on national TV and bluntly stated, "If you get vaccinated, you won't get COVID." This was fiction, even at the time, with fact checks pointing it out the same day. [2]
Then later, "If you are fully vaccinated, you no longer need to wear a mask.", also fiction at the time, as the fact checks the same day pointed out [3]. There was no science to back this idea that vaccinations significantly reduced transmission, as it wasn't part of Pfizers tests [4].
Apparently related, some FDA officials resigned [5].
Things stated as fact that never should have been, and no attempt to correct it later, is what did it for me. There were many more small things that, appropriately, hurt my trust, and I feel a little crazy that people don't remember much of this.
[1] https://www.businessinsider.com/fauci-mask-advice-was-becaus...
[2] https://apnews.com/article/joe-biden-business-health-governm...
[3] https://www.politifact.com/factchecks/2021/dec/22/joe-biden/...
[4] https://www.factcheck.org/2022/10/scicheck-its-not-news-nor-...
[5] https://docs.house.gov/meetings/VC/VC00/20220429/114682/HHRG...