if I had only known it would come to this, I would not have acted as I did; used to express regret for past actions or to reproach someone for their current predicament

Used to express that if one had known things would turn out this way, one would not have acted as one did; to regret a past mistake in light of present consequences.

used to express regret or hindsight about something that has already happened, often followed by a statement of what the speaker would have done differently