The good news is, since it's a feeling, it will fade naturally if you don't actively hang onto it and keep bringing it back to consciousness in order to punish yourself. If you believe the right thing to do was to intervene more decisively than you did, just resolve to take more decisive action the next time you hear something like that. Decide that learning that, was the purpose of the experience. Before learning that, you obviously didn't know it yet, and you did the best you could with what you did know at the time.