How do I do the straddle splits?

Okay, so I've been working on these for about 4 years already. I don't feel like I've gone down ANY lower. I'm a ballet/contemporary dancer (been one ever since)and I take classes quite often, so it really irks me that I'm not getting anywhere with this. I'm generally not flexible; when I do the butterfly stretch, my knees are way up. However, I was able to get the front splits on both sides. I just need a stretch that can make me get these splits!! My hips are really tight, I think. Please help meee