I recommend "The Mindful Way Workbook" - especially if you don't have a therapist at the moment. It's aimed exactly at what you are describing - the negative rumination that often results in - and is the product of - downward spiraling. Rather than talk yourself out of your feelings, which in my experience is quite difficult if not impossible (why can't I stop thinking about x! what's wrong with me! things will never change! I hate myself! etc. etc.), the goal is to return to the present moment and simply acknowledge and accept them. The methods have helped me a lot with anxiety.