When the analysis says you missed mate, it doesn't usually mean mate in one. It just means a clear path to mate; in this case mate in two.
Can someone help me?
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Instead of Bg5
Qxf7 - Kd8
Qxf8#
would have been mate.
Thank you both for clearing this up 
I need help understanding the end to this game. I missed the opportunity to checkmate 1 move earlier, but dont understand how it's a checkmate. In the analysis it says Qxf7 instead of Bg5 before Qxf7. What I dont get is whats stopping my oppenent from playing Kd8? It doesnt make much different to the outcome as Qxf8 would be checkmate but id like to understand how i went wrong.