I'm going to tell everyone my girlfriend cheated on me
Why do you think your girlfriend is cheating on you?
She keeps showing up late to dates with me and I feel she's been doing it on purpose to make me wait even longer.
You shouldn't say things to hurt your girlfriend's reputation. You don't have real proof she is cheating on you.
I mean I can't trust someone who is acting suspicious.
Just because your girlfriend is acting suspicious to you doesn't give you the right to lie about her. If you want to know what's going on then ask her.