I keep looking to see if anyone's been able to help you - I'm not sure I can! I play recorder, and the embouchure is different, but the amount of breath pressure must be fairly similar, I think.
Can you blow raspberries? Really sloppy ones? Try blowing a few, then bring your tongue inside your mouth and blow them there. It's a different feel from rolling an R. The amount of breath needed to get an inside-the-mouth raspberry is about what I need to get a note from my recorder.
I'm having trouble describing this because I never had any trouble with flutter-tonguing. Double-tonguing, now that's a different story...