Hey Tomer,
Try this:
Duplicate the layer you want to cast shadows, put a CC Radial Fast Blur on it, set the Zoom to “Darkest” and the blending mode to multiply.
Add an expression to the Radial Blur which sets its center position to the same position as your optical flare.
Add a further fast blur to soften the shadow edges and then adjust the Radial Blur’s amount until it looks good!
Hope that works for you,
• Archie •