Is this competely unreasonable of me?

I am in a relationship. Six months today. I am 18, he is 20. We say we love each other, but we both are on the same page when it comes to the fact that we probably wont be together forever.
As much as I am okay with this, it also scares me to death. So as a coping method I am flirting with this guy who likes me. I like him back but I have no intention of leaving my current boyfriend. This guy says he will wait for me.
Am I being selfish? Will no good come from this? I feel so juvenile and immature, but I don’t know what else to do. It’s so comforting to know that there will be someone for me when my boyfriend leaves me. But, is this fair to the guy I’m flirting with? Is it fair to me? To my boyfriend?
I just want to be happy.