hmm....what is it exactly that has made you think like this?...has something changed in her appearance i nthe last few months or have you just grown not to find her attractive anymore?..id say breaking up with her would be a mistake until you try some other options..my advice would be to suggest 'spicing' things up (if your relationship is sexual...getting her new lingerie,) or maybe taking her somewhere fancy for dinner so she'll make an effort and then compliment her lots..then maybe shell get the hint?
this is tricky, but whatever you do if you love her (or even if you dont for that matterP)please dont let on that you dont find her physically attractive anymore ..especially if worse came to worse you did decide to break up with ehr ...it would be CRUSHING