How to break away from a friend? (PLEASEEE READ FIRST!)?

So me and my best friend have been on and off again "best friends" with this other girl for almost 2 years. she's pretty bipolar- she'll be really nice for a while, and then she'll randomly start being REALLY mean and we'll get in a fight, she'll be mad at us, and she'll go tell everyone our secrets that we thought we could trust her with. we've finally decided that we absolutely HAVE to cut it off, but we know that the best way to do it is to slowly break away from her, instead of just confronting her. we've been trying to do that for the past 2 months. we've pretty much been ignoring her phone calls, texts, IMs, etc. but its not working!! at first we thought she just didn't get it, but now we're almost positive that she DOES know what we're trying to do and she thinks that if she pretends to still be friends with us that we'll go along with it, cause she's really desperate. she's also depressed and is suicidal at times. we're afraid of what will happen if we "the only things that are holding her together" (thats what she told us once) leave her/break away. we can't help her without hurting her. PLEASE HELP??? :( thanks!!!!!!!!!!!! <3