Im jealous of my friend

There is a girl i know at least for 8 years, i met her playing online and fell in love with her. The thing is she aways had a boyfriend, ever since i met her. She is with him to this day. When we met i were much younger, almost a child, but in time, i began to really like her, and she became the best friend i ever had. She is my friend to this day, and we talk frequently. She helps me a lot, gives me strenght to keep living, supports me in my projects and talk some sense into my head when im doing something stupid. She is almost aways avayable for me, and helps me whenever she can. But when she doesnt, i feel realy bad, more than i should. Anger, sadness, guilt... Specially when she is with him. Im having some trouble in dealing with this feelings. I already told her i love her, to let it out of me, and she kept supporting me as aways.