A word of advice don't have these types of conversations through text's, its either over the phone, in person face to face, or not at all. That way you can actually feel and listen to how she actually feels and what her emotions and body language convey.

As always V.Rossi(Hitch) is right...break it off or its only going to get worst and i guarantee you its not going to last...we all know its hard but it has to be done. She is in the prime of her highschool years and these last 2 years are the most wild years of highschool, filled with events. She doesnt seem to be taking this relationship seriously and you are putting in a lot more work than she is, shes ditching you to go out with friends; she has no respect for you. break it off before she does or its gonna hurt way worse...