Check out the video ... it's pretty dramatic. Shannen tells Orlando's MegaCon audience Sunday she's simply telling the truth about the situation "as opposed to the narrative that others put out there for me" -- this with her former costars Holly Marie Combs and Rose McGowan sitting supportively beside her.

She clearly gave a lot of thought to what she was gonna say ... even writing it down in pre-prepared notes which she read from during the "Who's the Boss?" panel.

Shannen makes it clear she and Holly weren't being "mean" when making the claims against Alyssa on her 'Let's Be Clear' podcast in December ... in fact, they say they later edited down the podcast to get rid of any extra deets that would likely cause more drama -- but remains adamant what remains is the truth "because the truth actually does matter."

Welling up with emotion, Shannen admits she felt compelled to address the matter following her stage 4 breast cancer diagnosis ... because there was no time more important than the present on getting the truth out there, at least what she considers the truth anyway.

SD says ... "There is no revisionist history happening in the truth that I know we told … There is no lateness to set, there is no mediator for months on end. I recall the facts as if I was still living in them."

Shannen reflects on the seriousness of the matter ... noting that while someone may call it all dramatic, it was an actual trauma she had lived with for an extremely long time -- and her cancer battle only served to heal her from the livelihood that was taken away from her and her family because "somebody else wanted to be No. 1 on the call sheet."

There's clearly no budging on Shannen's end -- and same for Alyssa, it seems ... who indirectly denied SD's allegations in another panel at the convention Friday ... saying she's "sad" by all of the "toxicity" surrounding the show. She added she was also upset that "people can't move past it" ... and that this was overshadowing the success of the show.