Let's take your questions one at a time, starting with the preliminary one:
What if it is just the Dunning Kruger or Impostor Syndrome making me think about retracting a highly cited paper?
Discuss this with non-coauthors, and particularly people who are not biased in your favor and have not already been "sold" on the paper, to see what they think. This is a bit risky though, because they could theoretically abuse your thrust and bad-mouth you elsewhere before you've done anything yourself; and you're sort-of committing to accept their advice. Still, another look-over of the results by someone other than the authors, and a consultation, is probably the best way to neutralize the effect of impostor syndrome.
What if my paper has a plagiarized section but the rest of the sections are awesome?
@Anyon already addressed this case. However - if it's just a paragraph without references, you might be able to get away with a corrigendum and an apology. Coordinate this with your co-authors and advisor though.
What should I do if the paper is misleading (say deliberately done due to lack of money to buy hardware resources like GPU or lack of time)?
The cause of it being misleading is not so important; it's the effect that's the problem. So, first double-check that it's actually misleading (again, possibly using a third party). Now, if the mis-lead can be corrected with a clarifying addendum/erratum, and is not properly false, then consider submitting that. At any rate, you should probably contact the editor and consult them about this situation. (Of course, they might just tell you "Just retract it, I can't be bothered with this stuff" and you would need to retract IMHO.
In the case of a retraction, try adding some minor additional result and republish the paper, done right, without the inappropriate section's content.