I'm on the same boat which led me to sign up to this forum. It's not as bad as it used to be but I still over-think pretty frequently - usually about past events or conversations I wished I handled better. My overthinking is rooted in negativity, regret, insecurity, and my discontent with my friends (I need to change my circle of friends ASAP).

Just like some here, distractions are great temporary fixes. Meditation works very well also. There is a link to this site I want to provide but am not able to due to not having enough posts (requirement is 10 in order to provide a link/attachment). However, type in "How to control the human body" in the search link above and you will find it. It's by Coma Patient 7.

There's a lot of info talking about how to control ones body (emotions, heartbeat, dreams, etc..). This can provide a healthy way to redirect ones focus away from incessant thinking and towards a healthy, positive practice.