<human>: Could you distill the essence of the text into 1-3 sentences?

I've been with my girlfriend for 2 and a half years now (we've lived together for about a year).

I've been having constant thoughts of breaking it off for a few months now, but I can never justify it enough to myself. I still love her, we never fight, our sex life is good, but I just feel so bored with our relationship and I can't see myself spending the rest of my life with her (she definitely does with me, talking about buying a house and getting married already... Maybe that's what scares me :/)

To make matters worse I've been starting to develop a (I hate the word but it describes what I'm feeling) crush on a girl I've been friends with for years. I would never cheat on my girlfriend, but I can't stop thinking about the other girl.

I don't think I could live with the guilt if I broke up with her, it would destroy her! Also I would appreciate any insights into how to handle breaking up with someone you live with... I just can't see how it wouldn't be a horrible experience :S
<bot>: feeling bored in 2.5 year relationship, still love her but can't see myself spending my life with her... What do?