Direct your attention to sleep. Whenever you go to bed and notice yourself actively thinking about something just direct that attention back to sleep. Think about it, it's the attention that keeps you awake. Note that 'actively thinking' is different than mind-wandering which is passive and actually helps in falling asleep. This will be difficult initially but you will get better over time at controlling attention. This is because attention is like a muscle so you need to train over time.
I have been doing this for months and it works.