I could not help but smiling when I read your question because I recognized some of my clients in your description of your girlfriend.
Some people feel alive when they stress. Even if they have all the proof that there is no reason to stress, they will find something to stress about.
The best way to deal with this is to not try and convince her that she has no reason to stress, but for you to stay calm about it. If you do need to comment, simply say that you cannot see a reason for stressing, but you respect her choice to stress and you love her despite and because of that.
When she becomes aware that stressing is a choice she makes, and that you are not going to change that choice for her, over time she will start to make other choices.
Behind this stress is a deep need for security and acceptance, and nothing you say will change that. She will change over time and realize that she does this to herself.
At the moment she is getting acceptance, albeit negative acceptance, from your efforts to convince her that she has no reason for concern. If you give her that acceptance by respecting her choice to stress, she will over time realize that there are different ways to get the same end result, and some of these ways are less painful than stressing yourself out.
To read more about dealing with relationship issues: