Wise words, especially the second paragraph.
Although others have suggested that the OP enjoy his youth, the old 'plenty of fish in the sea' style of argument, I would suggest that he take a look at himself.
While the girlfriend in question may well have checked out of the relationship some time ago, there may have been reasons other than attraction to someone else. The truth is, women tend to stray, or remove themselves from a relationship, when their needs - emotional and otherwise - are not being met.
In his OP, the OP himself has written - twice - that she said that he 'wasn't there for her'. If he is to rescue this relationship (a tall order), or ensure that a future relationship doesn't fall into the same trap, he may need to take a long, hard look at himself.
Again, wise words, and an opinion I have long held, myself.