Hello Friend, Good that you are seeking a help here. Your symptoms are related to anxiety. Anxious thoughts create negativity in your mind and also disturbance body functioning by causing sleeplessness, nervousness, palpitations, digestion issues and many more. You need to first figure out the actual root cause for such thoughts and the solution to overcome this. You may need a guidance of counseling support to figure out the reason and explore the  suitable solutions that you can follow. Based on the level of anxiety either self therapy or guided therapies or lifestyle change etc can be recommended.

The problem is not overthinking, it is just a symptom, the underlying problem seems to be something else. We need to understand the root cause as to why are we overthinking at all. Talking to a good psychiatrist/therapist will help you understand the same. About suicidal thoughts, thats an emergency and you should consider consulting a psychiatrist immediately for the same, whatsoever be the reason of such thoughts.