Just so you know, those feelings of passion will wane no matter who you are with. Ask any long time married person (>10 years). Other things do make up for this loss of early-days glitz.
You will end up a very lonely person, jumping from partner to partner if you go chasing that feeling all your life. What you are feeling are the hormones from a biological program designed to spread your genes. It has nothing to do with more evolved aspects of a relationship such as commitment and longterm growth.
That said, it may be you & your GF have simply outgrown each other. You started dating quite young. So, what you do should really depend on each of your other characteristics and how compatible you are. Good people and partners are very hard to come by, so be careful you aren't losing someone for the fleeting feelings of passion.
Read here, think about it, and then decide what you want to do. Good luck.
[url]http://www.loveforum.net/love-advice-forum/28296-stages-marriage-long-term-relationship.html[/url]
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh