Didn't read through all the comments. Anyway...

First thing u must know and always remember, we are all generally very fortunate to live in the society free of war and suffering all time, so we should be appreciate more than most majority in this world.
Secondly, I gone through the same thing, still having some nowadays but at least i consider it control-able.

Not sure about chemical reaction or hormones in the body or brain,
it is always best to seek consultation from counselor or psychologist but understandable that most people don't, even for myself.

Since u have the thinking to overcome it, and i assume no one around u knew about this, I guess the main method u can depend on is self help.

How: Basically, negative thoughts rise only when we are alone or when we have the time to let our thinking roam free. So u need to eliminate the spaces for ur brain to go wild. To do that, keep looking for friends and good people that speak interesting stuff, or join a community to perform some beneficial stuff like volunteer to help people, even if u feel lazy or dont feel like doing it, u have to force urself. Because its a battle between ur will and ur brain.
If u doesn't have a lover, look for one if u feel like u are lonely or need someone really close with u. But bear in mind that don't look for one just because u wan to fill ur emptiness, make sure u ready to commit into r/s and work out. Although this is not recommended since u having this negative thoughts in u now. Still, depends on ur will.

Not sure what u mean by doesn't work out when u pick a hobby. Perhaps u wan to let me know more.
Try out a balanced life, in the sense of working hard and play hard.
Since u are a working adult, u should find time to play.
Games or anything that make u feel fun and its wasting time. Yes its wasting time but it definitely beneficial to ur pleasure and enjoy.

Finally, don't keep urself in the environment where ur brain will keep going wild. You have to go outside and mix with people who said interesting stuff