First, do you like the way it breaks when it curves?? And what I said before IS how you make it break more. Unless your throwing motion hinders the way you throw the ball. I can only help you so much with words. So get me some video and I would be glad to take a look.
As far as how you hold the ball. First, holding the index finger up, puts all the pressure on the middle finger. Thats why it works. BUT good hitters WILL pick that up. I would stop that. there are many ways people hold the ball, to make it "break". The basic grip is held with the index and middle finger together, and the middle finger is placed on the inside/ of the "horseshoe" part of the seams. So when looking down at the ball, the horshoe part of the seams is open to you. You place you fingers on the right side (RHP)left side (LHP)- together with the middle finger inside the horshoe. Play with the placement of your thumb. But start with centering middle finger and thumb.
If you don't understand sorry. Just look it up.
I thew a knuckle curve, this has the same principle as holding the finger up, with out giving it away.
Keep asking questions. hope this helps