I have heart problems. My heart health?

This September I turn 30 yet I feel like I'm turning 80. I already have gray hair that I've had since I was 20. I have arthritis in my right elbow (I fractured it as a child). Now I have heart palpations and my heart races sometimes. Heart problems, early gray hair, and early arthritis all run in my family. It's just I want to behave like a 30-year-old I want to exercise I want to enjoy things. Seems everytime I get excited though my heart starts palpitating and/or racing. When my heart races or palpitates I sometimes have chest pain and/or shortness of breath. I've changed my diet to more healthy foods but I'm afraid to exercise as I'm afraid it's going to get my heart racing which I don't want to happen. Right now I only go for one slow walk once a day. I feel like I'm behaving like I'm 80 instead of 30. Do you have any advice on how I can improve my heart health? I just feel way too young for heart problems.