I’m sorry man I didn’t want to do this to you, but the love your girlfriend and I share is just something way too special for me to pass up. I didn’t want you to hear it this way.
For a real answer: provide us details, rather than consulting the fluther-based ouija board?
Usually, if you have to ask, you already know the answer. And why do you need a reason to dump her? If it’s not working out for you, that should be reason enough.