Unhappy relationship

I have been with my GF for a little over 3 years now but the last say 4-5 months i haven't really been that happy, its starting to really get to me now in ways where I'm not sleeping properly at night and im not 100% focused at work. I do still love this girl but i just don't love her how i used to, the only thing is i really don't want to hurt her as over the years we've obviously became best friends and i dont have the heart to break it to her. ALSO! Over the past 3 weeks ive found myself having strong feelings for another girl at work. I see her pass and look at me everyday and give a little smile then looks away, but not wanting to be that kind of guy i simply just give a friendly smile back and go back to work, im at the point now where ive forgotten what a good nights sleep feels like and i just need some sort of advice as to what to do. Thanks

You should have a frank discussion with your girlfriend and break it off with her gently. It's never easy to be the dumpee or the dumper, but it's worse in the long run to stay in a relationship that doesn't make you happy. Eventually you'll become resentful and you'll both be miserable. She'll get over it and you'll feel relieved to be rid of the situation.