Do I have to try to fix a problem myself before I can sue the seller over it?

Not exactly "fix it yourself first," but Ontario law does generally expect someone who has suffered a loss to take reasonable steps to limit, or mitigate, the damage rather than letting it grow unchecked and then claiming the full, avoidable extended loss. You're not required to personally repair every problem before making a claim against the seller, but reasonable, cost-effective steps that would prevent the situation from needlessly getting worse are generally expected of you.

A failure to take those reasonable steps can reduce what you're ultimately able to recover, even where the seller was clearly in the wrong to begin with — the mitigation principle doesn't excuse the seller's breach, but it does limit the buyer's recovery to what a reasonable response would have looked like. What counts as "reasonable" is genuinely fact-specific and depends on the nature of the problem and what options were actually available to you at the time. Before deciding to leave a problem alone and let costs accumulate, it's worth understanding how that choice could affect your eventual claim.