I will give you few ideas as an overthinker who often gets in his own way. There are many books, subs and ideas that can help you but first lets start with the what no to do. Your mind will want to do the same thing, using a different door. If its like to overthink, it will overthink how to live in the moment and how to not overthink. Counter intuitive if i say so myself.
You need a basic idea. A seed to work with where all the rest of the information connect to. That seed is " The mind is a tool. Recognize that fact! Its a tool, and it is Definity NOT you! What are you then? pure consciousness that few can begin to describe yet it requires no explanation " This means that if you are unable to put the tool down at will, then you are not using the tool. IT uses you
The way you learn putting this tool down is by observing it when it’s active yet serve no purpose. This includes, but not limited to, the 95% repetitive mental chatter. Make it a habit to observe those thoughts without judgment or reason. By watching the watcher, a higher consciousness emerges that is simultaneously you but also far greater than you.
Recognize that the disease that is overthinking is a parasite that will use every trick, method and way available to remain. The very fact you acknowledge and want to live in the moment, means that a glimpse of the real you is trying to separate itself from the tool, however this tool will not go down easy.
To move past it you have to abandon thinking, analyzing, strategizing how to move past it. You just do. With that being said, remember the basics in case things get confusing and complicated.
Your mind is a tool. Its supposed to be used when needed, if you cant put it down then it uses you meanwhile identifying as you. The real you, presence, spirit, consciousness, energy, vibrations or whatever term you prefer is beyond explanation. It’s what you feel when watching a beautiful sunrise or sunset that blows your mind away. Its the intense present you feel during high adrenalin situations that forcefully shuts the mind down anchoring you firmly in the present moment ( provided you are not panicking ) and its the feeling of being in the zone/flow where everything runs smoothly.