how do I tell her??

So I know this doesn't really fall under this topic, but it was the closest I could find.. well maybe about a year ago I had an abortion. I already have one kid and he is a handful enough. It was a personal decision I don't care if I get crap for it. But this is something I want to tell my best friend. We usually tell eachother everything and I don't feel like I have to tell her. I want to tell her. But before in the last when I told her I was having sex she said she didn't believe me and she thought I was lying. Then when I got pregant she said the same thing. I was 15 at the time so I don't blame her for not wanting to believe it. But now I want to tell her this, but like how. I have no clue how to even bring it up. And what should I say if she doesn't believe me becuase she still doesn't believe a lot of stuff I tell even after all that has happened...