The speaker has been hurt badly and is trying to make sure it never happens again. The image is simple but strong: a wound in the heart, closed up with wax seals, the kind once used to fasten letters and keep their contents private. Here the seal guards something far more fragile than a letter.
The last line carries the most weight. "Again" tells us this isn't the first time the speaker has bled, so there's a history of pain behind the poem, though it doesn't say what caused it. The speaker sounds determined, almost stubborn, as if closing off their feelings is a form of survival.
Still, wax is a flimsy fix. It's soft, it can crack, and it doesn't truly heal anything. That makes the mood guarded and a little sad: someone protecting themselves, but perhaps only by hiding the hurt rather than curing it. The pen name "Fallen angel" adds a note of loss, though that's only a hint.