One issue is that bad teachers or bad professors occur pretty much everywhere. They may be rare, but they do exist.
Oxford, Yale, and Harvard are reaches for everyone, and are particularly difficult for international students. I do know some people who got their bachelor’s in Canada and then did their masters or PhD or both in the US at very good universities (including Stanford and Princeton) so this is also a possibility. Also, as a Canadian, getting a bachelor’s in Canada plus a master’s in the US might cost less than just getting a bachelor’s in the US (and you will have the master’s).
I do not think that retaking will do you any good. I think that you just continue to take courses that make sense for you, do the best that you can, pay some attention to your budget, and make sure that you apply to some safeties in Canada.