How to handle cheating

So, my gf cheated on me about 8 months ago. With 2 people. I stayed with her because she wanted to change so I gave her the benefit of the doubt... here I am still not able to trust her. No matter how much i want to, I just can't. Im tired of my heart hurting everytime her phone bings. No matter who it is. I just hello thinking the worst and I can't stop.

Now, I know she loves me. She has shown me that... but I can't get passed it. And I mean 8 months later and I still hurt over it. And im tired of hurting. But I just don't want to hurt her either which is another reason I don't want to leave. ... any advice???