I'm having problems with sleep?

For the past 5 or 6 days I had incredible difficulty falling asleep. I've recently changed my bedtime hour, but I had this problem several times in the past without doing any changes to my sleep hours. What happens is that I lay in bed for 4 or even 5 hours without falling asleep. I thought this problem would go away, but it seems like it's harder than I thought. My mind simply stays very active after I put myself in bed. I can feel that I'm extremely tired, but my mind will simply not go to sleep. I'm stuck somewhere between being awake and sleeping, and I lose notion of time. For example, I decide to wake up so I can drink some water, thinking I've only laid for 2 hours but when I look at the clock it's 5 in the morning. It's horrifying. Now I get up at around 11 A.M, and my sleep hours are really messed up. I don't feel as rested as I used to be in my normal sleep. What can I do to sleep again? I tried everything: drinking milk, bathing before bedtime, listening to relaxing music, ventilating my room, nothing worked. I know I have to rebuild my biological clock and get used to sleeping and waking up at the same hours every day, but I must first manage to actually fall asleep. Is there anything I can do? Counting sheep or reading makes my mind even more active. I also tried not thinking at all (which seems impossible) or visualizing animals having wrong colors in order to keep both my brain hemispheres occupied, but nothing worked. What can I do? I don't suffer from any stress and I'm far from having big worries. I really don't want to suffer during daytime because of my nights.