Firstly, recognize the moment when over thinking occurs. But recognize it on the daily basis not just now and then.
Instead of focussing or living in past or future, live in present. Keeping yourself in present leaves no room for over thinking.
Letting go is not an easy task for everyone. We all want to take a control over those things which might even not in our hand. Instead of controlling, we should learn the skill of letting go.
Over thinking can be so strong and overwhelming, especially when it bring lots of emotions then just give yourself a break.
One of our major issues is that we are so much self absorbed. By being self centred, sometimes we trapped ourselves in over thinking. In that case try to focus on others who make you happy.
Meditation is very important for healthy brain. Meditation doesn’t mean a brain with no thoughts but rather relating them differently.
Be kind and gentle with yourself instead of getting harsh on yourself in over thinking. Normalize this over thinking thing. This shift in mindset may help you to deal with your issues without taking too much pressure.