DIAGNOSIS The variable speed trigger switch has failed. It's a common issue. The trigger works by sending more or less power as you squeeze; when it fails, it often defaults to either full-on or completely off.
FIX You'll need to replace the entire trigger switch assembly. It's a sealed unit, so you can't repair the internals. First, remove the battery. Take out the four long body screws and carefully open the saw. The trigger is housed in the handle. You'll see wires connected to it. Note or take a picture of which wire goes to which terminal (usually color-coded). Desolder or carefully disconnect the wires, remove the old trigger, and install the new one, reconnecting the wires exactly as they were. Reassemble the housing.
IF THAT DIDN'T WORK If a new trigger doesn't restore variable speed, the issue could be in the electronic control board (if your model has one) located near the trigger. However, on most P519 models, the trigger is the primary speed control, so this is less likely.
HEADS UP When handling the new trigger, avoid touching the electrical contacts with your fingers. The oils from your skin can cause corrosion over time. Just handle it by the plastic housing.