I sympathise with you too, I've had periods when I just couldn't fall asleep, no matter how tired I was. I find that I get tired/sleepy at certain times throughout the day, often around 2 PM, work or not - and also around 10 PM. If I don't go to bed at that time, I sometimes can't sleep for a few hours.

Getting into a routine of getting up and going to bed the same time every day helps, in my opinion.

I will not recommend sleeping pills on an every day basis, but one occasionally won't hurt. I tried some they give to passengers who has a long flight ahead, they work for 4-5 hours. The active stuff in them is called Zopliclone. For me, 1/2 of 7.5 mg works well.

Also, you can get various kinds of herb tea which will help, and Valerian (Baldrian) works for some too.

Sleep well!