I feel for you, I really do. I experienced pretty much the same thing with my mom and stepdad. My brother was my mom's favorite, and my sister was my stepdad's, because she was his only biological child. I made good grades, had nice friends, didn't do drugs or drink, and was an A, high B student who took all of the honor classes. My brother, on the other hand, did drugs, skipped school and dropped out his senior year. My sister ran away from home at the age of 15 to try and get married to her abusive boyfriend and later tried to kill herself over the same guy. Now, many years later, they admit that as teenagers, I was the easiest and best to deal with. I often think in my head that yeah, I was that way because you didn't let me do anything that would allow me to get into any kind of trouble.
It's really hard to deal with when you are experiencing it. I wound up spending a lot of time at my granny's house, because I didn't feel the overwhelming need to be perfect for her. I'm very close to her now, which is a great thing. I'm almost 29, and now, I'm kind of glad that I wasn't able to get into the kind of trouble my brother and sister did. They've had to struggle to work out their problems, and in some ways, at 22 and 26, they haven't done that yet. I also am rather proud of the fact that everything I have, I've earned. I didn't have it handed to me, as they did.
But, I will say that I've had to have therapy to work through some of this. I will give you one suggestion though, which I wish I had done when I was your age. When you feel like they are abandoning you or not including you in anything, go out of your way to make sure you are included. Just butt yourself right in, because I'll bet your parents might not even realize they are doing it or if they do realize it, they might think that you don't want to be included. My mom tells me that was the case with me. I hope things get better with you soon.