you are still young bro.. time to let her go and move on. i tell you this as someone who has as much as experience now in my 30's with dating and relationships as anyone you will ever meet. i've had 4 year relationships.. 2 1/2 year relationships... did the exclusive dating thing for 3-4 months, did the date 2 girls at the same time.. the 1 week fling, the 2 date fling... i've done it all.
if the sexual attraction isn't there then move on.. if you really love this girl then you can stick around but really what are you gonna do if you guys get married one day? have no sex? then get divorced. if she isn't head over heels attracted to you now whats gonna happen in 6 months or a year... or 5 years. she probably has lost physical interest in you, stuff like this happens... once the physical attraction is gone it isn't gonna come back.
chicks think differently than guys, she probably isn't banging another guy right now but she just is confused and doesn't know what she wants.. chicks change once you get my age, thats why like i told you above at your age just screw around and don't dare find anyone to fall in love with yet. finish your education/get your career/ business going. then when you get my age you can settle down, by then you will find a woman who has had relationships and knows what she wants, and if you have a great foundation and stability she will be yours and take good care of you and vice versa.
bottom line is when the sex starts waning away then a relationship is gonna be hopeless and not repairable. sex should impove as you get the know the person and learn what they like, it shouldn't get worse and the other person shouldn't ever turn the other down for sex.