Welcome to Fluther. This is a good question.
You’re young and immature yet, and that’s the main problem here. I don’t mean this in a pejorative way as if to call you “a baby” or anything like that, but… this is a fact that you may not realize yet. (Obviously you know that you’re young; you may not be aware that you are immature. It’s not a bad thing to be immature when you’re young. The terrible thing is to never outgrow that.)
You need to start growing up now.
Part of the way we do that is by developing our own self-interests apart from what our friends want for us, and very often apart from what is expected of us. It’s time to develop some of your own independence.
Make new friends, for example. Read more. (Not television; it doesn’t offer the same time for reflection and realization about how the characters you read about are the same as you in many ways, and also different in key ways, too. Not to mention that reading will help you develop your vocabulary and means of expression.)
You need to start reflecting on how to build your own life completely independent of others for when the time comes that you need to do that. That time comes to nearly all of us at one time or another. For example, your friend may move away, or you may, and then where would you be? You can’t plan your life around her and when she may be available. By exactly the same token, she can’t live her life around your wants and needs. That would be unhealthy for both of you.
The jealousy that you feel is fairly normal for someone who is young and uncertain of his or her own place in the world. When you start to develop your own spirit of independence, your own character, your own interests, then you should stop being jealous of how your friends spend their time away from you. Not only does it make you a better person on your own, but your friends will start to recognize your growth and you will become more interesting to them. (That may be kind of paradoxical, in fact, that you’ll be growing more interesting as you display more interest in the rest of the world and lose interest in “what everyone else is doing” and needing to be a part of that.)
Finally, you’re at an age when you might be developing feelings for boys yourself one of these days, or (this is possible, but I’m not suggesting that it’s likely or the certain reason for your feeling the way you do), your feelings may not run to boys. Perhaps what you feel for your friend has a shade of “desire” in it that you simply haven’t realized yet.
These are all feelings that are worth pondering and thoughts worth developing, but it’s up to you to do the pondering and developing. That’s how we grow.
When you can start to be happy for your friend because she seems happy, even if she has less time for you as a result, then you can know that you’re maturing. Let’s hope you can get to that point sooner rather than later.
Good luck with the process.