Hi Ann58, as an overthinker like yourself, I also found very difficult to sleep after receiving my diagnosis. What I found useful and do it until this day (just finished 4 x rounds of AC chemo)is having calming music playing in the background from the time I go to bed until the morning.
I know it is very tempting (guilty!) to get your phone out and start scrolling but try to concentrate on the tunes and take several deep breaths. If you have an eye mask, spray with any herbal scent such as lavender - the weight helps keeping your eyes shut and the scent is comforting.
I still wake up at night but it takes me less time to go back to sleep. I have discussed with my Oncologist other options down the track now that I have completed my chemo. Sleep plays a huge part on the recovery journey and you're not alone so discuss your options as you go along.
Good luck with everything x