Be willing to break it off. If you're Ok with it she'll wonder if she made a mistake. If you beg and complain and ask what's wrong, it will push her further away. Girls are attracted to strong men. Be strong and be willing to walk away. Don't treat her badly, just let her know that you respect yourself and respect her enough that you don't have to beg.
If she realizes she made a mistake now or in the future, all the better for you both. If you both go your separate ways, it was going to happen anyway so you're doing both of you a favor anyway.
Begging, pleading, and "trying harder" for someone who is losing interest will only push them away by making you appear less self-confident. If you want to find out for your own benefit if there was anything in particular that bothered her you should ask, but in a "gaining information for my future, which is bright" way, not in a "just tell me what I have to do to make you love me" way.
Confidence always wins out. Whether it has been my turd "friend" stealing my g/f in the past or whether it has been me who won out over other guys who were after the same girl, self-confidence has always played a decisive role in who wins out. My advice...