Make sure the cutter, is straight up and down,aligned with the ruler. some people tend to turn the cutter,and it tends to cut a little under the ruler..which would make the piece a hair short.
don't be reaching very far either. if your holding the cutter properly and it has a sharp blade in it..you should not have to use so much pressure.
my favorite all time ruler is a 4 1/2 x 18,
"Creative Grids" non slip ruler.
they are a good investment. I wish i had known about them when i first started. My ruler collection would of been alot less.
You can get them at Local quilt shops and on line. They are worth the price.
that size is not only easy to use..it is easy to take with you to classes and such.
I have a smaller mat, so i can move the mat, instead of the fabric. 18x24. wait for it to go on sale or use a coupon at Michaels or Joanns or Hobby Lobby.
Good luck, and keep us posted! i know you will get better as you practice more. next to your seam allowance, the cutting is most important!