I'm in the same situation with anxiety. My boyfriend used to suffer from it too so I know he understands what I'm going through, I just feel like I'm letting him down or dragging him down too. The best thing to do is speak to her. Explain how you feel but make sure she knows that deep down you trust her. The best she can is offer reassurance. It may be a bit strange to her and take a while for her to come round but she will understand and support you.
I always get paranoid my boyfriend is cheating on me, even though I have no reason to and I trust him. It helps me to just send him a message saying I love you or something. Waiting for the reply is the worst but when they reply back it means the world and reassurance comes back. I know it sounds silly but little things help. Speaking to someone about it helps too!
Don't worry, your girlfriend will understand and support you through it!