I dont know how to help

So Ive been trying to look up ways to help my girlfriend with her anxiety and I stumbled upon this page. I hope I can get some advice on how to handle some of the situations that are putting a lot of strain on my relationship from people who either have anxiety, or have a loved on with anxiety.
I just dont know what to do when she has a melt down. When she starts panicking I dont know what to do. I try to comfort her the best I can but she starts yelling at me. I come from an abusive family, I tend to shut down when someone starts yelling. I try explaining this to her, but she cant seem to control it when she gets that hyped up. Things just tend to fall apart from there, I cant deal with being yelled at for things out of my control, she gets mad cause Im not helping her, I get mixed singals cause sometimes she wants me there and sometimes she wants to be left alone and I never know which time is which.
I love my girlfriend very much, when things are good they are great. I just need help on how to handle this?