The correct answers are I have loved you since the day we met! and I have been thinking about you constantly lately.

Certain verbs are "stative" (they describe a state of being, emotion, or possession) and are rarely used in the continuous form. Love and believe are stative verbs, so they must use the Present Perfect Simple (have loved, have believed). Think is an active verb here, so it works perfectly in the Present Perfect Continuous to show an ongoing recent habit.