I am feeling selfish and I hate it?

One of my best friends just got engaged and I am happy very for her and her fiance. I like him a lot and he treats her well. But a part of me was saddened by the news. She will be getting married soon and things will change and I don't want them to, at least not as fast as they are, we only graduated high school a little over a year ago. This is probably just part of becoming an adult, but I don't think I am ready for all of this yet. If she is married she will not have as much time as she use to or the same priorities as she use to. I feel very selfish thinking like this and I know that I can not keep living in the past and have to accept changes. I don't want to be sad about this news, I love her and hopes she and her future husband have a wonderful life together, but when she told me the news part of me could not help but feel a bit disappointed. Is there anyway I can stop thinking of myself and just be happy for her?