Am tired of living with my girlfriend,how do I tell her I want to leave?

My partner and I moved in together 3 months ago. It was a huge step for me living with someone other than my parents for the first time. But I've now become uncomfortable and unhappy in the house with her. Our sex life has dried up and I crave my own space and own bed. It feels like I'm a lodger renting off her, rather than a place we share. I do care about her but I'm constantly coming home from work and feeling miserable. I've found the most amazing flat to move into if I wanted to but when I tried to tell her my thoughts and be honest she got extremely upset and begged me to stay. I don't know what to do. My heart says stay but my mind says go...I can't stop thinking about having my own place! Help!