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Some lifts are really tough to get right by yourself too, even if you are being coached by someone.
You can sort of do the right movements, but still engage the wrong muscles.
If your proprioception is telling you that this feels like how you were taught, that's what you're going to continue with (and that proprioception is going to drift over time too).
Then it's tough to recognise that you're gong wrong, let alone fix it.
On the other hand, it feels to me that the majority of people I see in the gym have poor form and do dangerous stuff but it's not often somebody gets hurt. I think that's sort of the problem....you can get away with something for a long time right until you don't get away with it any more.