Sorry this may be long and rant-ish but I need advice -_-||
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In my circle of friends I have to: talk, be happy, interested and never unsocial, just to keep them with me,( as opposed to being alone.)
Unfortunately I have no one else to go to, no other friends. Im in my last year of comprehensive so everyone is already friends with each other. Plus it takes me ages to make friends with people in real life, too long.
But my friends don't care. For example:
One day I was hella depressed - I didn't talk to anyone that day. None of my friends tried to help or comfort me. They left me alone, making it worse. Later my friend told me that they were close to saying that they would no longer be my friend because I wasn't talking to them, for one bloody day. And that our 4 years of "friendship" meant nothing.

Another thing is birthdays: I remember everyone else's and I don't like people being left out . But as soon as it comes to mine, January the bloody 5th didn't even exist. All I expected was for people to remember and maybe a card would be nice. In the end, only one person remembered, the others didn't bother. (I had one card, a month later).
I try and do nice things for people: give umbrellas, open doors, give directions, help people out etc, and all I want is a thank you or just some sort of acknowledgement that I'm there.
The sere amount of times people interrupt me when Im talking, despite letting everyone else speak, is enormous: It happens all the time. They just don't care that Im there.
I just feel like a fucking door mat. Walked all over and hardly noticed.
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What do I do? Please help, I cant get new friends and these are the only people I have, and they're toxic. I don't really want to be alone again.

You really are in a bind. It sounds like your friends aren't friends worth having and yet you feel like you can't leave for fear of being alone. You try to be nice yet nobody notices you or even acknowledges your kind acts. If this is correct then we are in the same boat (or at least sailing the same sea).
I really suggest finding some worthwhile friends, but if you feel like that isn't an option, then do something that catches the eye. Have some kind of impressive talent or hobby that makes people watch. This isn't a perfect solution but it should at least speed up the process of meeting new people and help you get into other groups.