I used to have a great bit of trouble cutting until I started using the June Taylor Cut Shape ruler. It is a slotted ruler, and you just put the rotary cutter in the slot and follow the opening. It has saved me. I don't cut anything without this ruler. The first time I used it was a bit scary, as I just barely understood the instructions on how to fold the fabric to get what I wanted, but I got used it very quickly.
You might google it and see what you think. This ruler is really the only one I use for cutting out quilt pieces.
Dina