My best friend has a new best friend?

I've been best friends with this girl, Cathy, ever since we were toddlers. This year (first year at secondary school) we promised to stay best friends all the way through it, but things just aren't working. We aren't in the same class for most things, but we are for PE. It was a shame when it happened but I started making a whole new group of friends within classroom hours and Cathy normally did loads of lunchtime clubs so that was okay. We still phoned each other lots and had sleepover and spoke on Facebook and MSN, so it's not like we weren't close anymore. Then a new girl joined our school and was in most of Cathy's classes and a few of mine. I noticed her and Cathy were becoming really close, hanging out everyday and sitting together at lunch most days. On Facebook they done edits of them together and were married on Facebook. I decided to become friends with Mel (the girl) and she seemed really nice at first but then Cathy said I was stealing all her mates away from her and I should get my own. I told her to get lost, it was a free country and she didn't seem too upset, just said she was sorry and everything was alright again. A few days later Cathy invited both me and Mel to hang out at hers one Saturday. We did that and we had a good time until about 3 hours in and Cathy started changing. She kept whispering and giggling with Mel and I felt really left out. So I told her I didn't feel well and went home. When I got home I went on Facebook and saw what Cathy's status was. That I was fake and a rubbish friend. I wrote back 'Say that to my face Catherine' and she said she was sorry that she got hacked but it was kinda true. I ignored it and decided to be my own person and step out of Cathy's shadow for once. The next few weeks we barely spoke and Cathy became so close to Mel they could almost be twins. I was good friends with a girl called Louise who was really nice but Cathy had fobbed off as stupid. Then about 6 weeks after the Facebook incident, Mel came up to me in French (she's in my French class) and said that she was sorry for the past few weeks, that her and Cathy's friendship was falling apart. I decided at PE that I would speak to Cathy, she spoke to me and it was as if we were the same as always, NicoleAndCathy. But then her new friend Marion came and I knew things wouldn't ever be the same. That was about five months ago at the start of the year and me and Cathy have only ever said 'Hello' to each other in the corridor. The endless hours on the phone or online with her stopped and she became best friends with Marion. Mel and I became really good friends together with Louise, but no matter what, I really miss Cathy. Any ideas on how to become best friends again?