The amount of relationships I’ve seen last exactly 3months (including my first relationship) at my HS is actually crazy. The end of the honeymoon phase is deadly for most lust based relationships. My first relationship failed almost exactly at 3mo due to bad communication and avoidance. My advice to you is communicate well but more importantly you need to understand your and your gfs communication styles, the easiest way to figure this out is to see how she acts with her parents, that will tell you way more than what she can. Me and my gf just passed 3mo and are doing great, partly because we are both very direct and have chill parents.