Hi wills11,
this sounds like the place I was in a couple of years ago. I really needed help to break that cycle of negative thoughts. It took a combination of medication - to reduce the depression and give me a chance to have positive thoughts, and therapy - that explored the long term reasons for my anxious and depressed state, and CBT exercises - that created another pattern and trained me to recognize what I was thinking.

It is a huge change to make, so get all the help you can before you get in really deep. Depression is serious.
Whether any of those approaches will be right for you I cannot say, but try whatever works for you, and do get help with it. We don't try to tackle cancer alone, and depression can be just as much of a battle, and just as lethal.