[Marty is holding his forehead with both hands now.]

Oh, no, no, no, Doc, say it ain't so. I-... I... Oh, man Doc.

[He's panicking a little now, the gravity sinking in. Swallowing, he squeaks a little and covering his face all sweaty.]

This is my fault, I shouldn't've ever gone to 1955... or 1885... or 2015... Doc, I destroyed the universe.

[His parents, his siblings, Jennifer, his guitar. All gone! He saved up all summer for that guitar.]

I th-think I need to sit down, Doc.