How do I get rid of PTSD from being cheated on?

My girlfriend of almost a year cheated on me a while back and I haven't been able to move past it. She's been doing great we even shared a phone for a while. But I still cant shake the thought that she might do it again. I catch myself thinking about it and wanting to throw up. Not to mention I have severe separation anxiety from her now and when she picks up her phone my heart races and I feel sick and I start trembling. This is my first serious relationship, I'm 21 and I've never loved anyone more, please help me..I don't wanna lose her and i have no family or friends please she's all I have...