She did not believe me

Hi, male (18) here. Last year I tried to come out to my best friend (female). We went to a park and I slowly tried to get to the point. After some time I told her that I have something important to say and told her I was gay. But she didn't took it serious and thought I was joking. I have no idea why she didn't believe me. My entire courage to come out went away and even after some tries to convince her it didn't work and I became too scared. Now one year later she still believes that I'm straight and I really want to tell her but have no idea what to do. Did anyone else ever had something similar happen?