Codependency Support Group
Codependency is defined as someone who exhibits too much, and often inappropriate, caring for another person's struggles. A codependent person may try to change, or feel shame about their most private thoughts and feelings if they conflict with the other person's struggles. If you are on a journey towards self-love, this support group is for you. Join us and find others...
I try mindfulness exercises. They can teach you to focus your attention on the things that you want, tuning out some of the "brain chatter" associated with the over thinking. the website calm.com has some interesting free exercises that you could try.
Regarding over-thinking about a specific person, this is tougher. I have the same issue sometimes, wondering if I should do/not do, or say/not say something to get the responses I want from him/her. In these situations, I try to remind myself that this is my vice and that positive long-term relationships do not come from trying to manipulate the other person's feelings nor do they come from trying to marginalize my own feelings.
Good luck as you think (but not overthink) more about it.