I feel your pain. I have been in your exact situation. Not once. I also had a long distance relationship where we were together for 1 year before it turned into long distance. And still it didn't work out although we've tried to make it work. All I'm saying I can relate to your situation.

It doesn't look too good to me. If you don't break up in the summer you will after you guys move away. In this case the rest of the summer will be hell for you.

The core problem in your relationship is that you want to settle down and she wants to have fun. Afterall she's young. I really don't want to keep repeating what others said about the age thing but generally an 18 year old girl is not relationship material period. I say generally, because there are exceptions, but she doesn't act like one.

The best thing you can do is tell her that you feel she is not contributing to the relationship as much as you do. You have named a few examples here, you can tell her that. Try not to get upset. I know people here say don't act needy and all that shit but if you have to pretend in a relationship you will go crazy and snap at some point anyway.

Tell her that if things keep on like this (only chatting online, spends more time with friends than you, not enough sex etc.) than maybe you two are not compatible and maybe you are better off without each other. If she tries to make it work, great if not then it's not worth staying with her.

I hope you can sort this out, this is very likely a breakup eventually. However how long you drag it out is only up to you.