Play with her heart strings

The next day you put on your best clothes, groom yourself perfectly, and put on some cologne. You make your way to Stag's Cafe super early and rush in to find Tori cleaning a table. You approach her.

"Tori, I have something I have to say."

"Oh...um ok go for it," she says confused.

"From the minute I met you, I knew you were special. I knew that you were going to be different than the rest of the world. You were the first person I could open up to, and the only person who gave a damn. I have never been invisible to you, and you don't know how much that has meant to me. I tell you that to say this...Derek has been nothing but a ruthless tormenter to me my entire life. Him being back in town has been hell for me and it's only been a day. I know I told you you should go with him tonight, but just know that when he's trying to charm you off your feet that that man has hurt me more than anyone else. Don't let him get to you."

Tori just stands there. Completely speechless. Time to stick the landing.

"Don't worry. I'm not here to stop you. I just needed you to know how I truly feel on the matter. I hope you don't think less of me," You say. Then you turn and leave.

Perfect. If this works the way you planned, she'll be obsessing over that all day, and when it comes time for the party, she'll be riddled with guilt, confusion, and feel too conflicted to enjoy herself.