Not attracted to partner anymore?

I have been with my girl friend for a lot of years (we never got married as it just went that way) for the past few years i have not found her sexually attractive at all and i won't engage in sex or anything to do with that. She loves to cuddle and kiss, but i don't. I love her as a person and we can laugh and get on without a problem. The thing is, i don't want to do things with her, go on holiday, go out, etc. It's ending up where i'm sat in at weekends and not being bothered about doing anything and i think i'm wasting hers and my life. I've often thought about leaving but i can't imagine how much pain that would cause her and i would lose my best friend. After all, it's just down to sex and is it that important? I think it is, but i also think i may be being shallow, I have to do something as i think i am wasting both our lives, anyone been through a similar thing?