if your gf lost weight/ at least started getting toned up would you stop thinking about this other girl? if you think you'd still be pining after this other girl i think you're just plain losing interest in her altogether. you've been together for a long time so she feels very comfortable with you. you should also feel comfortable enough to talk to her about this. don't mention the other girl but say that recently you think that she doesn't seem to make an effort and you really miss her doing so. if she takes it on board then do it together [home-cooked meals, long walks..] and if she gets all immature about it then maybe she is infact not the one for you. but tread carefully around the subject of a girl's weight, i'm sure you know that anyway.
They called us a dead generation,
They told us that we wouldn't survive
They left us alone in the maelstrom
As you can see we're all clearly alive.