You can know exactly why you are sad and still not have let yourself be sad. You can trace an anger back to its source, name the pattern it belongs to and explain it with real accuracy, all without the anger ever fully arriving. Understanding a feeling and feeling it are not the same act, and for some people the first has become such a reliable substitute for the second that the difference is easy to miss. Telling them apart is the beginning of choosing between them on purpose.
Insight answers why, experience simply is
Understanding is interpretive. It steps back, finds the logic, places the emotion inside a framework and tells you what it means. Experience is direct. It is the heaviness itself, the heat itself, the longing itself, before any of it has been translated. Both are forms of knowing. Antonio Damasio's work suggested they are meant to inform each other rather than compete, since the felt, bodily signals he called somatic markers are part of the information clear thinking actually relies on (Damasio, 1994). The trouble starts when one of them consistently arrives first and ends the other.
Insight tends to resolve, feeling tends to move
When you understand a feeling, there is often a small sense of completion, as though the matter has been handled. When you feel a feeling, it moves through you, sometimes slowly, and it does not always resolve on schedule. This is one of the clearest ways to tell them apart. If something clicked into place and then felt finished, that was probably understanding. If something is still working its way through you, you are likely in the feeling itself.
You can describe it fluently and still be outside it
A telltale sign is fluency. If you can narrate an emotion with clarity and even elegance while feeling oddly untouched by it, the narration may be functioning as a buffer. Being articulate about a feeling and being inside it are different, and the more skilled you are at the first the easier it becomes to mistake it for the second.
Understanding can be a way of not staying
There is nothing wrong with insight. It becomes a problem only when it is used to leave. When discomfort shows up, understanding offers a fast and dignified exit, a way of rising above the experience rather than remaining in it. Some things, though, cannot be thought through to completion. They have to be felt first, in their full weight and strangeness, before any understanding becomes useful.
Neither act is superior. A life lived only in feeling would be overwhelming, and a life lived only in understanding would be strangely thin. The skill worth building is noticing which one you are doing, especially in the moments that matter most, and being willing to let the feeling have its turn before the explanation arrives. The feeling is not the preliminary stage before the real work. Sometimes the feeling is the real work, and the understanding is what becomes possible only after.
References
Damasio, A. R. (1994). Descartes' Error: Emotion, Reason, and the Human Brain. Putnam.