Can't say this is a "solution", but negative thinking is habitual. Our brains are hardwired to reward repetitive thinking by strengthening the connections we use the most. This is the same mechanism that allows us to get better at tasks through practice.

Unfortunately, our brains don't differentiate between positive or negative thought patterns. It doesn't reward us when we think good things, while punishing us for thinking bad things. it just makes it easier for us to think about things, that we think about often.

This means that negative thinking reinforces negative thinking...making it easier and easier to have negative thoughts. It becomes an automatic function that our brains execute like muscle memory.

The only way to stop this from happening, is to force your brain to think about something else...and repeat it often enough, that the new pattern takes priority over the old one. Eventually the new pattern will become as automatic as the old one, and will no longer require effort to execute.