Definitely look into therapy and self-care. Even within your post, you automatically apologize for any mistakes you might have made (it's not bad, and of course it's good to follow the rules of any online community!). But there's no need to worry or be apologetic when you don't even know if you've done anything wrong. I point this out because, well, it's a behavior I've noticed in myself as well

Also, try to establish some boundaries with everyone who comes flocking to you to vent. Tell them that although you care about them, hearing about their concerns only fuels yours more. Let them know how stressful being the "receiver" is all the time. Perhaps you could ask one of them if you could vent to THEM.

The "catastrophic thinking" is also something I relate to. You should look up cognitive distortions (sometimes called Automatic Negative Thoughts, or ANTs):
PsychCentral Cognitive Distortions
Cognitive Distortions printable PDF

Of course, it's important for you to be aware of your girlfriend's emotions if it seems like she's leaning towards self-harm. However, I hope you can find a good balance of caring for her while also caring for yourself and not getting too stressed out.