Maybe try some guided meditation. It's hard for your brain to overthink stuff when you're listening to a guided meditation or some really nice, relaxing music.
I love Enigma or anything with soothing nature sounds.
Also, try this guy:
Brian David Phillips: Waking Dreams

Click on "MP3" on the left hand side of the site,
Then click on the top link, "Guided Imagery"
Then click on the "Guided Imagery MP3's by Brian David Phillips
Go to the second session and download all 12 (plus the 2 intro's) for a total of 14 guided imagery sessions for relaxation.

He's a hypnotist and an entertainer, but his stuff is really good and it's free.
Perhaps this could help you with your meditation...

Good luck.