wahnsinnig literally translates to "insane" or "mad". While it can mean this, it's more often used colloquially in German to express an extreme feeling, much like "crazy" or "incredibly" in English.
In the song, the singer says, "Und es macht mich wahnsinnig" (And it's driving me crazy). He uses this intense word to describe the overwhelming frustration and anxiety he feels from his partner's silence and emotional distance. It's a perfect word to capture the raw, emotional turmoil at the heart of the song.
Ever felt like you're talking to a wall? That's the feeling at the heart of this song. The singer notices a change in their partner, telling them straight up, "Du bist anders" (You are different). The problem is, the partner won't say what's wrong. They just stay quiet, which drives the singer crazy because, as they admit, "ich kann das Schweigen nicht so gut wie du" (I can't handle silence as well as you).
The whole song is a desperate attempt to get an answer. The singer keeps asking, but gets nothing back. They can see something in their partner's face but can't figure it out, which leads their mind to the worst places: "Ich frag' mich ob du Angst hast / Oder ist da jemand anders" (I ask myself if you're scared / Or if there's someone else). It's a raw plea for honesty from someone who is being shut out.