Hi, can this be related to stress or lack of satisfaction of what you are doing?
What are Your working habbits? It can be usefull to do some calm activity before sleep (not near any electronic devices)
Headache and eyes hurt.. are You working with computer? What about trying to give your yeas a break more frequently or do some joga for eyes (artofliving.org/yoga/health-and-wellness/yoga-eyes)?
Finally i think it is beneficial to eat less baken food, less meat, less milk based foods.
Have lighter diner maximum 1,5h. before sleep
Regarding main question "Why can't I go back to sleep", it feels like an nervous issue and feeling stress, like you are telling yourself (subconsciously) that i did not do as much as i should or i have alot of work, or im missing something and i need to do something.. it can be subconscious feeling, so one should focus on what she/he is inputting in her/his subconscious mind.