Gedankenkarussell literally translates to "thought carousel" or "carousel of thoughts." It's a vivid compound noun that perfectly captures the feeling of having thoughts spinning endlessly in your head, especially when you can't sleep.
In the song, the singer describes being awake at 3 AM with a "Gedankenkarussell," indicating a restless mind filled with swirling thoughts and emotions about the person they miss. This unique and descriptive word is much more evocative than simply saying "many thoughts."
"Hallo du" starts with a very specific kind of sleeplessness: it's "Drei Uhr nachts" (three o'clock at night), and instead of sleeping Daniela is stuck in a "Gedankenkarusell" (carousel of thoughts), her heart going too fast. Then a text comes in from the person she can't stop thinking about, and she almost doesn't believe what it says: "Dass es dir genauso geht" (that you feel the same way). That's the whole setup, and it's a good one, because the chorus that follows turns the plainest greeting in German into a full confession. "Hallo du" just means "hello you," but here it's followed straight by "ich liebe dich" (I love you), like the small talk got skipped entirely.
The verses keep circling back to how much this is costing her: "Du fehlst mir viel zu sehr, mit jedem Tag noch mehr" (I miss you way too much, with each day even more). She's not hiding the anxiety in it either. "Echt verrückt" (really crazy), she calls it, before deciding to just write to him and say it anyway. By the end the song has stopped being about longing and turned into a plea, repeated twice like she needs to make sure it lands: "Vergiss mich nicht" (don't forget me).