What is the ruling on me supplicating with my own personal supplications in the Witr prayer behind the Imam who is supplicating, while I follow him in the rest of the prayer's actions?

The correct action for the ma'mum (follower) when hearing the imam's supplication during the Qunut is to say "Ameen" to his supplication. If they do not hear it, then they should perform Qunut for themselves. What the questioner did, intentionally performing Qunut for herself while hearing the imam's Qunut, does not invalidate the prayer.