Sullen load is full, so slow on the split
The sex is really good, though impersonal and emotionless. It is implied that the narrator is still lonely despite being in a sexually-gratifying relationship.
The speaker could be describing the load as sullen, meaning what is being carried is bad tempered, or gloomy. Slow on the split could mean the the lover took a long time to finally end the relationship, as the speaker had mentioned wanting the lover to end it earlier in the song.
Vernon is describing his current behavior as sullen, saying his load is full meaning he’s carrying so much baggage he doesn’t have room to help his current girlfriend, and that he is so slow on the split, meaning that his recovery from splitting up with his last girlfriend is slow. Note, the lyrics here should probably be corrected to have a comma before sullen. Sullen, load is full, so slow on the split.
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