Best way to comfort someone having a panic attack?

Girlfriend has really bad panic attacks in which she cannot control her breathing and sometimes passes out completely. This happens even when I'm there and being who I am I don't take not being able to help very well so when I realised that I couldn't seem to find a way to stop them completely (she has only ever passed out one time with me there) it started scaring me.
Normally what I do is I lie her down on her back and lie next to her on my side. I drape my thigh across her lower abdomen and hold both of her hands while resting my head very gently in the crook of her neck so she can feel my breathing. I take as much care as I can to avoid chest area upwards to face in fear of impeding airflow. I try to talk to her to get her to concentrate but often I have to stop when I see how scared she is because hearing my voice crack and having me sobbing into her neck probably won't help at all (especially considering I am a very calm and level headed person and that is so out of the norm it might scare her further)

tl;dr I need to be able to help my girlfriend when she is having severe panic attacks what is the best physical reassurance and what can i say
Updates
+1 y
I have some people saying that I should avoid close physical contact and some elaborating on ways I can better the comfort and help I can provide by this?
I'm assuming that I should just ask what she would rather I do but since I have never not done this she might not know what it's like and be too scared to try it?
Best way to comfort someone having a panic attack?
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