I don't have a significant other like this but, my mom is like this. Totally negative. And, her perception of everything is "right" in her eyes.

Here lately I've learned to take a deep breath and kind of tune her out when she says things I don't agree with or are totally negative. I am not sure that would at all be a healthy way to deal with a significant other though.

I've also pointed out to her the positive side of things at times or just said, "Everything happens for a reason", or "Did you think of it this way".

I find her negativity to sometimes be draining on my positive nature. I totally empathize with you and hope that your T can help resolve the problem. Kudos to you for asking for help. Thats a grand step on your part.