I suffered from this for years. Fell asleep easily but awoke most nights and then couldn't get back to sleep. My mind was going flat out. End up with chronic fatigue.
You are already doing all the good sleep hygiene things i.e. dark room, no caffiene writing stuff down etc etc and that helped me a lot, but didn't totally alleviate the problem.
You should also look at eating foods that increase your melatonin levels, i.e. high in tryptophan e.g. turkey. You could also try eating different foods prior to sleep and at different times, as depending on your metabolic type, some people sleep better having eaten protein/large meal and others light meals/carbs.
If you are happy to take supplements, then 5HTP is the best. It increases your serotonin level and therefore indirectly your melatonin level. But is a very natural substance.
The above is all helpful, but to me it sounds like your sympathetic nervous system is over stimulated, i.e. you are suffering from excess stress of some kind. This could be physical, emotional, work, environmental etc etc. Only you really know.
So you need to engage your parasympathetic side more. I found that meditation/breathing helped a lot if I awoke. You should also tailor your diet to support your parasympathetic side.
But ultimately you need to get the root cause of the stress.
And if you are drowsy mid afternoon, then sounds like your blood sugar levels are unstable, so worth looking at the balance of carbs/proteins etc in your diet to stabilise them.