Please help me, Worry marring my relationship!

Please, someone help me. Several months ago I began pursuing a man and eventually my efforts were rewarded and he now adores me more than life itself. At first, for several months, I feel beside myself with romantic glee. I’ll admit, I first only pursued him because I was lonely and he was the first one to notice me. But soon after that the relationship deepened and I couldn’t imagine myself ever loving anyone else. But since he is so much older than me and lives in another country, and for the fact that most people except my fiance and my closest friend think that I’m still only “in love with being in love”, everyone’s been telling me it won’t last. And now, to my own shock, I’m beginning to doubt my capacity to love him forever!! I’m as physically and (I think) emotionally attracted to him as I always was. I’ve talked to him about it and he tells me he believes that we can love each other forever and that my fears are groundless. Oh, I hope he is right!!! I know in my heart of hearts that I want to love HIM and ONLY HIM for the rest of my life. So Why am I suddenly wondering weather I can or weather I will? I’m so frightened that this mindless fear will ruin the most enchanting relationship I have ever known. How do I get this nagging fear to leave, and enhance the bonds between us?