There are quite a few different views on when you should tell your girlfriend that you love her (or not tell her at all, and play around “who tells it first”). However, from my own perspective, if you really do love your girlfriend as I do love my lady, the best thing you could say her is “you are the love of my life“.
The toughest (for you guys, who are just trying to sound right) and the easiest (for us guys, who are really blessed with special lady who we really do love) thing will be to sound like we really mean it.
But I do love her. And she is the love of my life. And she should really know it. Your girlfriend, should really know it as well. So, what stops you from saying it? You know you love her.