Emotional restraint becomes strength when expression would create damage without creating understanding.

Saying everything you feel is not automatically honest, and holding back is not automatically dishonest. The test is what the expression would accomplish. If it would help someone understand you or fix something between you, say it. If it would only wound while leaving both of you exactly where you started, then holding it is not repression but care, and it takes more strength than the release would.

Before delivering something sharp, ask what you want the other person to know or do afterward. If you can answer, find the version that gets that across. If your honest answer is that you want them to feel bad, wait until that has passed.