Well... you can never faint with anxiety. I learnt that recently and found it somewhat useful. You only faint when your blood pressure drops. When you are anxious it increases.
Anxiety symptoms, however bad or scary, do go away. They don't last forever, although sometimes it can feel like it.
They won't kill you.
To be honest, in answer to 'how bad can anxiety symptoms get?', I don't really know. However, they don't last forever. The worst panic attack WILL end.
Sorry. I know this probably isn't much use but I wanted to put something positive. Plus, knowing that you won't faint can be useful.