Why do I have so many nightmares?

I've been having very vivid and disturbing nightmares every night for the past six months. They are always about the same themes, but in different situations. I have recurring nightmares about swimming away from something, being late, being chased, missing the bus, false awakenings, ghosts, people who threat me awfully, people who laugh at me and act like they hate me, strangers, and much more. I've tried everything, including reduding stress. I haven't gone through any traumatic experiences lately either. But nothing works. I don't know why I have so many nightmares, and as I said before, I've tried everything. What should I do?

I too have nightmares although mine are starting to bother me less and less. I think part of the reason why people get stuck in a cycle of nightmares is because we tend to focus on them, thinking about them and giving them attention even after we wake up. We're thinking before we go to bed, 'hope I don't have a nightmare tonight' and then what ends up happening is that we end up having one because it's on our minds. Lately I've been having nightmares about serial killers, haunted houses, and abandoned psychiatric wards... and I haven't watched American Horror Story in years -_- BUT they are getting easier to deal with, especially since last year I was having dreams that I was being attacked by vampires in my sleep. THOSE were scary.
Anyway I think this is where lucidity comes in because becoming aware that none of it is real helps to make it less scary.
As to what you should do, I don't really know because I'm not all that experienced with any of this yet. Maybe try and reflect on some waking life fears that you have that might be influencing your subconscious to produce all these ideas in your dream. You probably need to change the way you're thinking. For me a big part of this was accepting that I'm not always going to be in control of life or my dreams, and it kind of helps.

I hope you find peaceful sleep.

Try to make nightmares a positive experience. Try be entertained by them or interpret and learn from them to figure out their meaning. Try to become lucid during them turn the dreams around into happier experiences. Pessimism fuels nightmares while optimism puts them out.