Right, plainly speaking, I'm that girl- the neglectful girlfriend. Me and my boyfriend have been together for 4 and a half years, living together for two and distanced at uni for two. I get bored quickly of routine, and I'm just going to share how I felt in that situation, so you get another perspective.
I would suggest that for her the spark has gone out and she might have a school girl crush on another guy- these won't be real feelings as such but more grass-is-greener yearning. I'm not saying that she's cheated as such, but if you guys aren't having fun (why aren't you going out and getting drunk with her?!) and she's having fun with other people (men) its natural that she's going to prefer their company.
The natural response is to confront her, but if I were you I'd take it the other way and rather than just discussing it to death just change the way you act so you're more fun- maybe take her out on a super special date, inject some passion into your relationship.
You sound a bit like my boyfriend, and for want of a better term, he goes through "boring" phases, where we get a little stale. Some times you need to jump start out of your routine, splash some cash, and make her feel special, not in a "clinton cards- I wuv yooooou" kind of way but in a "super horny ***** me now" kind of way.
Does that make sense? You can shower as much love on her as you like, but all she'll really want is a bit of fun.
Sorry if I'm off the mark, it's just I've been in this situation from the other side and constant talks of "are we going to stay together" and "I wuv ooo this much" sort of stuff kind of drags and just pisses me off after a while.