What do I do!!!! I need help!!! My relationship is going sour!!?

So... for about 3 years now I've been consistently able to lie to myself and keep saying "It will never happen"... I actually made myself believe it... and about 2 weeks ago these feelings came up again and I tried my hardest to push them back... I'm failing :(... for about 3 years now ive been madly in love with my best friend from before kindergarten... you know about the time when those feelings occur... and I'm in a relationship... I keep trying to push my emotions back but they just keep coming back stronger and stronger... I dont know what to do... I want to tell my bestfriend I love her yet I dont want to hurt my girlfriend... ITS KILLING ME! D':my emotions are trying to blow my insides out and i cant hold them back any longer... its getting increasingly harder because my best friend and I are starting to talk a lot more again... and my emotions for her are getting harder to even hold onto... even my dreams are complete and utter chaos about this.... no matter how hard i try to work this out myself i cant :( i feel pathetic that i cant work this out :(