Overthinking

Question for all of those in a relationship/situationship...I over think a lot when it comes to my SO and its a huge problem for both of us. Its usually over some dumb shit that I know I plant in my own head. He plays no role in it and doesn't do anything for me to think this way. I blame it on the fact that I'm a Capricorn and everyone I know that is a Cap and does it too. I told myself to change and I don't act on it anymore. I think the world of him and want to quit. How do I stop doing it? Can anybody relate to me?