Oy... I'm glad I'm not thirteen anymore, but some things never change.
My advice is actually drawing on the fact that she's really a good friend. Personally, from what you've said, she sounds pretty selfish. But assuming she's not, I would say find a time that you can do something, and make that time count. Pick your moments, and tell her, upfront, but not in a confrontational tone, that you haven't gotton to spend much time with her lately, and that it does kinda bother you that you feel like you're being shifted to the side. It's hard to keep that last part from sounding accusatory, but that's the only way it'll work, just keep your voice down and look her in the eye, and try convey that it really saddens you, not that it makes you angry. It wouldn't hurt to remind her that since you're 13, these things will come up (you know they will, it doesn't get easier for a long time, sorry to say) but you want to stay friends, so you think that
both of you will need to keep it in mind and work hard to stay friends.
Unfortunately, like I said, I think the girl sounds selfish. If she is, I think you just need to do the hard thing and let go. It's hard to do (I've had to do it lots of times), but one way to look at it as there are two kinds of good friends. There's the kinds who are your bestest friends and help you through a particular time in your life, but those friendships aren't meant to last forever, and there are a few (and I'm talkin a VERY VERY few) that are friendships that will hold together for years and years.
Coincidentally, after 24 years on this planet, I have two friends who have always been there. I've always got lots of friends, but those two friends are the ones I know I can call on when I need them, and they're the two that I call my "best friends". I talk to one of them about once a week on the phone (she's in another state). The other, we've gone months without talking, but when we finally do, it's like the time slowed down, and it's just like it was when we were twelve years old.
There are LOTS of people that I've been friends with who were my best friends at certain parts of my life, but I have zero contact with now. Those friendships I'm so thankful for, they got me through some tough times, they were great friends and great people, and I would LOVE to see them again, but they weren't the kind of friendships that last forever. There wasn't any "breaking up"... it started much the same way you were talking about... "Nope, I can't get together, I'm really busy, but we really need to do something soon..." or "I just haven't been able to call lately, with work and the kids..." It doesn't cheapen those friendships, they weren't "lesser" friendships... they just were a different kind.
I think most of the people you are friends with right now are of the second kind. Heck, most of the people
I'm friends with are of the second kind.

It's hard when you're parting, but there will be other friends who fit who you are now very very soon.
If you really think this girl is the kind of friend that you want to be friends with through the good times and the bad, if you think she'll be there for you, then say something to her, definately (I've had to do that with one of my two friends... It was awkward, but because she IS the kind of forever-friend, I could and not feel bad about it). If you think she's someone you can look back on and remember when you were kids and were friends, but you don't see in the future that she'll make the effort to be there for you, and you dont' think you'll really be there for her (it's okay if you make that decision) then don't worry about it, just try to make it through and not get your feelings hurt because she's moving on.

and I think my answer is longer than your "ranting" problem, but I hope I've been able to help a little bit.