Insomnia can be caused by many different factors, including hormones, psychological reasons and medical conditions. It seems like there is a connection between genetics and insomnia as well, but scientists do not yet fully understand the connection. You can read more about this here: http://www.scienceblog.com/cms/genetic-link-found-between-stress-induced-sleep-loss-and-intrusive-thinking-21941.html
http://www.ehealthmd.com/library/insomnia/INS_causes.html

No matter what the reason, there are some general tips that can help. I understand that you tried many of these, but let me suggest some more. I had trouble falling asleep sometimes when I was younger so I know how frustrating it can be. Once you start having trouble falling asleep, the worrying of it happening again can itself cause sleep problems. To prevent this, avoid going to bed before you are tired and don't lie in bed for long period at a time if you can't sleep. Instead, get up and read a book or something else for a few minutes and then return to bed as you start feeling sleepy again. Try to unwind before going to bed: avoid making phone calls, working, having conversations about stressful topics, paying bills etc. Instead, do something relaxing such as reading a book or doing yoga, meditation or relaxation practices. Make sure to get some sunlight in the day time to signal to your body what the time of the day is.