My boyfriend is scared that I will break his heart again. How can I reassure him that I will not?

My boyfriend and I first fell in love at about 16 years of age. He asked me to marry him at about 17 and instead I went off to college. He was devastated for years and would contact me from time to time.
We are beginning to plan our future together. He told me the other day that he is scared that I will break his heart again. It has been a few decades and I was surprised to hear that it still affected him.
Is there anything I can do or say to reassure him?