If racing/anxious thoughts isn't the cause, you may have great success with the military sleep method: https://health.clevelandclinic.org/military-sleep-method

I was sure the version I'd read months ago, had you tense up the muscles before relaxing them section by section; but I could not immediately find any variabts which mentioned that part. I used to do the very lite version, counting my breaths. Worked pretty well, over the years - after getting my head sorted out first - I've built a habit of falling asleep in 5~15 mins. Used to often take an hour, occasionally 2~3.