Do you think I should start seeing other people?
What makes you want to see other people?
I've been in this relationship for so long
Wanting to see other people is fine. You should make sure your current relationship is over before seeing other people, because it is rude to cheat.
I've been with her since I was 16 and I'm 18 now. I feel that we aren't really headed anywhere and are kind of just wasting our time.
Yeah, it's okay for you to feel that your relationship isn't working for you. Have you mentioned this to your girlfriend? You two probably need to discuss this before you go out and cheat on her and get her upset.