How to get over this feeling of being lonely?

i dont know why i always feel lonely. i have friends. lots acutally. i have a nice house and a supportive family. but i always feel lonely and i swear im depressed. i listen to really slow depressing songs ( sweater song by hedley) and i always in vision myself having that person to be with and share everything with. my best friends at the moment is becoming closer and closer to a girl she just met (their like best friends, probally better than her and i are now) and i know i shouldnt have but she left her facebook on my computer so i read an inbox message between the two of them. It was a letter from her new bff and it was all about how much she loved their freidns ship ( she was like "i know im pretective over you but i just have never had someone i can share so much with. and your so mature for your age. I know im guarded but i feel like i can open up to you") i was really deep. Her new best friend is 4 years older. Im 15 and so is my best friend, but her friend is 19. and i feel like im being left in the dust while they keep getting closer. They met through her brother and so my best friend has become part of the older clique. I donno why i always feel so lonely. but when i hangout with their older group of friends, it just doesnt feel right because im the noob that nobody knows. My friend is starting to pick her new friend more and more because shes older, she drives and there just best friends now.but my friend isnt being mean to me or anything, like we still hang out but its not the same. When were together she is constantly texting her other friend and when she was at my cottage she kept texting her saying howmuch she wanted to go home. its really frusturatiing and im really lonely because I want to have what they have soo badly but it just never happends. Wow. this is more like a diary entery but whatever. Any suggestions of how to deal with it or the lonelyness ... thanks