Up until a few years ago, maybe even six months ago, I would've said this friend was my best friend. We've known each other since we were in our early teens and although we've had periods where we haven't lived in the same country (like right now), we've always been really good at keeping in touch. We would text every single day and often talk on the phone every other day too. But over the past few months especially, I've had the feeling that I'm being phased out. It was nothing dramatic, but just every time I'd message I'd get a reply a little bit later. Now it's reached a point where my messages will go days without being answered. We're on other chat groups together and she's messaging on those groups still, but just not to me. As far as I know she's perfectly happy - in fact, she's recently married to a guy I set her up with 
I really don't know why I'm being phased out. I've always had a sense that she's more important to me than I am to her, but this feels almost like a deliberate attempt to put distance between the friendship. I can't think of anything I've said or done to upset her, and if I ask I'm afraid it'll seem needy or clingy. Is it best to just let things drift off because it seems like that's what she wants, or do I keep trying?