My best friend is calling him and making sure he's okay instead of me

You're going to have to elaborate a little as what you have here doesn't make sense.

Your title and then the body of your question together make things confusing.

I broke up with my boyfriend last night and my best friend was texting me how upset he is, and how she's talking on the phone with him, and she's making me feel so guilty, and bad about because he's talking about how he's going to commit suicide, and he cut himself and sent her a picture of it.

She's more concerned about him even though I'm crying and have a history of cutting too.

I don't know what to do because I woke up to 6 long messages from him about how he loves me and I'm perfect n that he doesn't want to live without me and he's not going to unless we date again help please ! I'm only 15! I don't believe he can love me.

Tell your friend to stop enabling him to try and control you by threats. Tell your friend that if they care for you, don't talk about him to you again. Tell your friend, if they want to talk and deal with your ex, do it.

You stop contact, don't talk to him, don't talk to people sending your messages. He will use threats to make you jump to his every desire if he sees it will work.
If he is telling someone he is going to cut or kill themselves, tell your friend to report it, but not tell you, by reporting, they get professional help, by telling you, they are working on his side, not your side.

Some times breaking up with a boyfriend, also means that a few friends have to go, if they can not support YOU.

He will stop at nothing to get you back, so tell your friend to stay out of your business, and you both ignore him. Your friend is scared of this guy, and what he can do. He is trying to scare you both, but don't fall for that.

Threaten to tell his parents if he doesn't back off, and be ready to follow through if he continues.

Thanks, ill try but I really don't want to lose this friend and I keep getting more texts from him saying I have to love him and that I will one day

Your friend is as caught up and as much a victim as you are. He doesn't care, he is manipulating you both.

Don't let him get away with this, help each other against him.

I got his friend to talk to him, and he freaked out at me saying if I want somebody to tell him somethig than I should get my best friend too, cause hed rather hear I from er, but the thing is she can't keep a secret and everybody already knows about it all but he's like blind or something and thinks he can trust her . But I can't tell him he can't cause he will tell her

Maybe its time to get a responsible adult involved to sort this mess out.