Mental stress is impacting your physical health so first you need to address your overthinking. Overthinking means that you're living in the future, Practice Being in the Present moment. Be mindful of each and everything you are doing right at this moment.
For example: suppose you are eating chocolate, focus your attention on the sound of the wrapper while opening it, and then focus on how chocolate is melting slowly in your mouth... Similarly, be mindful of whatever you are doing in your day-to-day routine.
Deep breaths can change your state of mind immediately. Breathe in through your nose and out through your mouth. Make your exhale longer than your inhale. Write down your problems, which gives you a chance to make sense of them and come up with a solution.
Once you realize there's a solution, your overthinking will lessen. If your depression is severe then you require medications along with counselling.