What's a friend ?

I'm one of those friends that have to keep reminding people that they're still alive. If I didn't call my friends frequently I would never be invited to go sleepover or go to the movies with the rest of them. Me and one of my friends have practically the same style, and I guess it's cool, not when we accidentally buy the same thing though. Whenever I tell my friend how I feel about her buying the things I told her I was going to get, and how I never would get invited to hangout unless I called she just says that she's so sorry and didn't do it on purpose. But this happens all the time, and now I think it's time for me to reconsider me definition of friendship. It seems like all my friends are talking about each other behind their backs, and I hate that. I want to hang out with my best friends all at once without having to worry about a disagreement or someone whispering in the other's ear.I have 3 main friends, and one of them I know I can't trust, but she's still my bff, and I always give her the benefit of doubt and just try not to tell her too much . Then there is another one who I have to call at least once a week, or else she would totally forget about me, but out of all my friends, she is the best. My other friend is just like me with attitude and sarcasm, sometimes that's annoying. And I have this feeling that she lies a lot, it's hard to pretend like I believe her 100%. Whenever I'm totally bored and my internet's not working, (which happens frequently) I usually call the one that forgets about me, and she starts telling me about what a great week she's had and it makes me depressed cause it's always with a different friend. Of course she tells me that I can't sleep over her house or she can't sleep over my house because her mom says no, but in a couple of days when I call her back she tells me about how she spent the night over our other friends house and how they hung out together for 2 days straight. I used to bring up the fact that she told me she couldn't hang out with me cause she was busy but then she would hang out with someone else ,but it's just useless now. And now that I'm staying back in 8th grade I'm going to be left out even more. I'm so confused about my friendships. When I start to question if they are really my friends they tell me stuff like they were planning on staying back in 9th grade so we would be together again, and my worries go away. But that confusion is always in the back of my mind, I don't know what to do.