Worried about relationship... what's wrong with me? please answer!!?

My boyfriend and I have dated for almost two years. Next year, we will be seniors in high school. He wants to go to college in california, and I'm leaning towards the east coast. We've talked about it, and we know that our relationship won't last if it's long distance (like most high school relationships), but we want to stay together in senior year because we don't wanna face it alone. Ever since we've talked about it, I haven't been able to be as open with him. I've been kind of flirting with other guys, and he's told me I've been more distant. How can I make our relationship in senior year be really good when I'm so worried about being hurt in the future? Also, why do I want to flirt with other guys?