Im unhappy with my relationship, what should i do?

my relationship with my girlfriend has been going south recently... shes not happy with me anymore i feel, and always focuses on the bad. there are other girls that i love to talk to and know would make me happier. its my last summer before college and i just want to have fun. weve been dating for almost 2 years and we barely even talk anymore. everytime we hangout it just turns into sex. its nice. im not complaining. but its not fun anymore. im too young for this. HA