I'm very much in love with my girlfriend, and I honestly believe I could spend the rest of my life with her, she's the best thing in the world and the most motivating person I've ever known. Though I don't doubt my ability to spend the rest of my life with her, I'm having trouble accepting the reality that we might not and it hurts.
Live in the moment!!! If she makes you happy ...don't take it for granted by worrying about what may or may not be.
I'm not a buddhist, but I think there is a notion in buddhism that the fear of suffering creates even more suffering. so this might apply in your case. realistically, most relationships break up sooner or later I think getting stressed about the possibility that this could happen would just make things worse though many people never meet someone who they feel like they want to spend the rest of their lives with. you have a special gift, something that you can enjoy. the memories that you have with the one that you love can last for a life time. if you do eventually break up, hopefully you will be able to handle it in a good way, not have an ugly break up. it could be that you may separate, but then get back together later or maybe you will just stay together hope that everything works out great!
I know the feeling. I feel this way about the guy I'm with now. Things are very difficult in our lives right now and I'm not sure if things will really last but hopefully it will. The only advice I can give is to just enjoy it now, and do what you can to maintain the happiness and make it last. Don't stress too much about the rest.
