My girlfriend seems like she doesn't care?

Me and my girlfriend have been going out for 8 months. I really love her and i always have. But lately it seems like she is using me just to like have someone to kiss and call her boyfriend. Her and her friend think this boy i know is hot and when he was at the beach like 50 yrds away she was just like staring at him and stuff. And she tells me not to talk to other girls and when i do she tries to make me feel bad about it. Yet she talks to alot of other boys. Like today me, my friend, her, and her friend went to the beach and it seemed like she was flirting with him and stuff. and she just takes me for granted sometimes it seems. Its really bothering me to the point where i cant even bare to talk to her sometimes. What should i do?