Some people thrive on stressing about everything. Rather than reassure them, tell them you respect their choice to stress but there are other choices available.
To read more about dealing with relationship issues:
My girlfriend stresses all the time about her finances and her grades at university. She is very intelligent – and this shows in the grades she gets. She is also good with money and as a result financially secure with her income.
However, no matter what I do, she disagrees. I have quoted her grades to her, and I have worked out her budget to prove her financial situation to her, but all she said was that I am trying to prove her stupid.
How do I get her to stop stressing over nothing?