Why is it so hard to stop focusing on something you can't change?

For six years, my best friend and I were inseparable. We were always together. Our senior year, we had talked about how we were going to get an apartment together and go to college together. I thought that she was always gonna be there and I had a friend for life. Before we could do any of that, a month after we graduated, she got married! It was so sudden and she had only dated the guy for 2 weeks. My other friends and I never even met him until after they were married. No one understands why she decided to get married. They went to the courthouse and got married. Her family offered to give her the wedding of her dreams. They told her she could have it annulled and they would give her any kind of wedding she wanted. She refused. I have asked her why she did it but I never get a straight answer out of her. I never see her anymore... It is so hard to go from seeing someone every day to never seeing them at all. Her husband is very controlling. Now, she is pregnant and due any day now. It so hard to know how to say goodbye to someone that was such a big part of my life.. Is something wrong with me? I mean, I just can't be okay with this. If I thought she was truly happy, I would eventually come to terms with it...but, as of now, I have no clue how to cope. Guess it's just one of those things in life that I'm just gonna have to deal with... Any advice?