Ausmachst comes from the verb "ausmachen", which can have several meanings, but in this context, it means "to bother" or "to matter". It's an interesting word because its meaning shifts significantly depending on the context.
In the song, the line "Dass du mir gar nichts mehr ausmachst" translates to "That you don't bother me at all anymore" or "That you don't matter to me at all anymore." It perfectly captures the singer's struggle to convince herself that she's over her past love, making it a key emotional point in the lyrics.
LEA keeps circling back to someone she's trying to quit. She opens with the line she'll repeat all song, "Immer wieder denk' ich / Immer wieder an dich" [I keep thinking, always to you], then lists the small tests she keeps failing: walking past his front door without checking his window, scrolling through old photos and pretending they don't do anything to her. "Ich hab' gedacht, ich hab's geschafft / Dass du mir gar nichts mehr ausmachst" [I thought I'd done it, that you don't matter to me anymore], she sings, right before the chorus proves her wrong again.
The bridge names the trick she's using on herself: "Wo keine Liebe ist / Verpasst man auch nichts" [Where there is no love, you don't miss anything either]. Talk yourself into feeling nothing and the loss can't reach you. It doesn't work; the last thing she sings is "Immer wieder an dich" [Always to you], same as the first.