My husband also has a hard time understanding this. He would get frustrated and tell me things like, you're just doing this to yourself! When I found this site, I printed out info from the Panic Program and had him sit down and read the introduction and some of Week One and I told him, this is what I am going through and even though I know you can't understand this because you don't go through this yourself, all I ask is that you give me some time alone so I can deal with this and support when I need a moment. I also told him that if he can't understand then to please just pretend to because it only makes it worse when he tries to motivate me to "just get over it."
Be as open and honest with him / her as possible. They get frustrated because they don't understand and that frustration turns into anger towards you. Try to help him / her understand. Start the Panic Program (it is free) and share the info with him / her so he / she will have at least a better idea of what you're dealing with.