Hi I'm Jessie Villalobos there's this girl named Katie and she's the most amazing girl I met I would love to ask her out but she already committed to someone and I respect their relationship but I don't know what to do :(

Simple really.
Back off as she's already involved.

You do nothing.
When you go to someone's house and you see something you really like but you know it does not belong to you, what do you do?