Get out of a relationship w out destroying the girl. Other chick involved too...ugh

Really need some perspective here guys. I'll make it short.

There's this great girl (call her Crystal) I've known for a long time and 3 months ago realized that we're totally into each other, something fierce. Problem is, I took a new girlfriend (call her Kendra) about a week before Crystal and I figured this out. So the whole time with this new girl I've been just thinking about and talking to Crystal.

Crystal's pissed bc I'm still w Kendra and talking to her at the same time. She didn't say that but she's pretending it's about some other bs I know isn't a big deal. I need to break up w Kendra but this girl is like self destructive and I think in a month or so I could do it but she's in the middle of some crap right now I'm afraid she'll get wasted and hurt herself somehow. I just want to do it the right way.

Q: What is the right way to break up w a girl who is totally not expecting it?
Q: I see Crystal every couple weeks or so bc of mutual friends. She's a tough girl and I can't push her, she'll just push back and further cement her "decision", which she doesn't really want. I need to finesse this thing. How do I go about this? Freeze Out? Explain that I know what I'm doing wrong and trying to get out of it, or do I just start over with the whole attraction thing from the very beginning, hopefully after I take care of things with Kendra?