A lot of guys are going to read your OP and see "she hasn't felt sexual for a while" and run on that by saying it's just you she isn't feeling sexual with, when it really could be her.
People like to oversimplify the lack of sex by drawing conclusions that it's caused by a lack of attraction that you won't be able to get back, and she's on the way out the door. She could be depressed. She could be thinking about a guy she saw while she was there. Who really knows?
This could very well be true, but it could also be that she just isn't as sexual as a person as you would like. That's a dealbreaker on its own for a lot of people, myself included. You need to decide how important that is.
You are 18, and you've only been in a committed relationship with her for 8 months. That might be a long time for you, but it isn't a long enough time to really outweigh what's going on currently.
You should not refrain from breaking up with her just because she's your only option, and she is your first. There are millions of girls out there who will have sex with you if you game them right.