I think I might be slipping into depression. How to prevent it?

For some time, I'm waking up at night crying without a certain reason. I also feel like I'm an "observer" whenever I'm with other people.
I relive all unpleasent situations in my life and remember constantly what mean someone said to me years ago.
I keep thinking about my childhood and what went wrong back then despite being 28 now.
I'm giving myself hard time for my past decisions.

How to stop?