Drawing from my personal experiences, these are a couple of things that happen when you’re in a negative headspace:
You attract the wrong things
The wrong relationships, the wrong jobs, the wrong people. You most likely will see yourself as less than who you are and believe me, people can sense that and take advantage of it. Just because they can, and just because they’re human.
People see and treat you the same way you see and treat yourself
I got into an argument with a person I know recently and during the argument, the person said some really vile things. Luckily, I was in a place where I was paying very intense attention to my negative thought patterns and behaviours so one thing about the incident really stood out for me; the person was simply mirroring all the vile and negative things I thought about myself.