How do I prove that a professional's negligent advice actually caused my financial loss in Ontario?

You need to show, on a balance of probabilities, that but for the negligent advice, you would not have suffered the loss - meaning you have to prove both that competent advice would have been different, and that you would actually have acted differently if you'd received it. This second part trips people up: even where an expert agrees the professional's advice was substandard, you still need to show you would have made a different decision, not just that better advice was theoretically available.

Reliance is central to this. Evidence that you asked specific questions, expressed particular concerns, or would clearly have taken a different path with accurate information - documented at the time, not reconstructed afterward - carries far more weight than a general claim that you "would have done things differently." Expert evidence is usually needed on two fronts: what competent advice should have looked like, and what the realistic outcome would have been had it been given. Where multiple factors contributed to your loss, such as market conditions alongside the bad advice, the professional's negligence still has to be shown as a real, not merely theoretical, cause of the specific loss you're claiming.