How do I forgive myself for staying silent at work?

You stayed quiet when you should have spoken up, and since then you cannot quite meet your own eyes. That regret sits heavy because it touches your sense of who you are. But notice something first: the fact that it bothers you this much tells you your conscience is intact. A person who did not care would have forgotten by now. You have not, because you are better than that one moment. You cannot go back and speak the words you swallowed. That is done. What you can do is let the regret teach you instead of just punish you. The point of guilt is not to sit in it forever. It is to change what you do next time. Carry the lesson, not the endless sentence. That is what turns a failure into something that made you stronger rather than smaller. So decide, plainly, who you will be the next time it counts. If it is possible to still say something, say it, even late. If it is not, make a quiet promise about the next chance, and mean it. You are not the person who stayed silent forever. You are the person deciding, right now, not to do it again. Let that decision be how you make peace with the last one.