I'm not sure how we can move forward without some degree of empathy; "Yes, you got the short end of the stick, but how about if we try such and such to ameliorate the impact of the past on your present".
I don't think you are advocating sweeping the past under the rug, I'm just saying that telling a person who is still feeling the sting of a perceived slight (real or imagined) is unlikely to result in moving forward.
Our ignorance of the cultures (positive and negative) in parts of the world where colonization did not happen is motivated by something less pernicious - people are parochial, and European culture in particular took a fairly strong stance that despite knowledge of the civilizations along (e.g.) the silk road, they were of no particular significance since they didn't have (Jesus|Bach|Newton|Galileo|etc.)
You ask "to what end?" I would say the end has multiple components. One is that history rhymes and so if you want to understand the future better, understanding the past better will often help with that. Another is that cultures themselves carry the past forward for amazingly long periods - the English have still not really abandoned the Norman conquest of 1066 as a socio-structural signifier even though it was nearly 1000 years ago. The Hopi still have many stories of things that occured in their world 600-1200 years. If these historical stories are inaccurate, a culture is doing itself no favors carrying them forward. And similarly, a culture that carries such a story as a tale about injustice is not done any favors by being told "ah, fuhgedaboutit".
What? The Rennaisance that grew birth to the Modern West was an intentional attempt to revive and surpass the ideals of Old Rome and Greece. When they excavated the Pyramids, many in the West took to adopting parts of Egyptian Culture for legitimacy, the Washington Monument being one prime example. Imperial China was seen as stagnant, but they certainly were respected as highly civilized and organized. What special qualities they gave themselves were their flexibility, rationality and technological superiority, which is not entirely wrong in the battlefield.
This "central myth" you are saying Europeans told themselves sounds more like a fictional strawman to attack and is contradictory, especially in the context of OP's point towards the attitudes held by the people currently attacking Western ideals, not defending it. It's not really refuting the point either that the world pre-1945 was a brutal place, and EVERYBODY was trying to conquer and dominate each other, it was just the West was the strongest of them all and won at the end.
That's why if you solely focus on the West as opposed to understanding the general context of the world at the time and critiquing equally those other culture, it calls into question whether one really cares about these shared ideals of anti-imperialism or if it's just an excuse for nationalist grievances that they weren't on the dominating side. And you know, in Turkey, in China, in India, that kind of mindset very much is the case. It's not that imperialism was bad, it was only bad because it happened to them. For those they conquered, it was glorious event to be valorized.
The myth is "we bought civilization to places that didn't have it". And that is absolutely a lie. There is a second myth that is particularly applicable in the Americas, which is that Europeans discovered a land that God intended them to have dominion over (essentially ignoring or belittling the existing civilization that was here). Neither of these are fictional strawmen - they are real and documented positions found through the writings of European explorers and American settlers and leaders.
It remains puzzling to me why settler colonialism (the dominant, though not only form of European expansionism) was not common in either the pre-1492 Americas or in Asia. The cultures/civilizations there certainly were expansionary but rarely seemed to feel the need to replace existing populations with their own. Whatever the reasons, the results are wildly different.