How do I get back at my cheating girlfriend?

I just found out that my girlfriend has been cheating on me for the past couple of months. We have dated for 3 years and my heart is broken. I have written proof of conversations that she has had with the other guy. As you can probably guess, I’m a little bit pissed off! What are some good ideas on how to break it to my girlfriend that I know she’s a cheater? How can I get back at her. This might all sound terrible but anyone who has gone through this knows how I feel. Thanks guys