How to overcome sadness and repeated negative thoughts?

Hello doctor,

I am facing some ongoing mental health challenges and would appreciate your guidance. Throughout my life, I have experienced recurring negative thoughts and feelings of sadness, which I used to associate with a strange sensation in my chest during my childhood. At the age of 13, I started having panic attacks, but they subsided after a few months. However, when I turned 17, I developed serious health anxiety and was convinced that something was wrong with my heart, leading me to stop eating out of fear of allergies. I had numerous trips to the emergency room with a rapid heartbeat (180bpm), but all tests showed normal results.

Now, I am 19 years old, engaged to a wonderful partner, but I often feel like they might be cheating on me, even without any evidence. I also find myself repeatedly searching the internet for answers to the same questions, which gives me temporary relief, but the negative thoughts return soon after. This has affected my interest in activities and studies; I've lost motivation and cannot sleep well. As a result, my academic performance has suffered, despite being a bright student in the past.

I am hesitant to try medication, so I am wondering if there are any alternative approaches you can recommend to help me manage these repeated thoughts, feelings of sadness, and other related symptoms.

Thank you for your support.