Don't quit.....Perhaps it is your seam that is giving you problems. I have a terrible problem with keeping seams at an even 1/4 inch. A piece of moleskin stuck 1/4 inch away from your needle helps (or any one of a lot of ways on this site to keep that seam even.)
I use two rulers to cut, which is a tad more accurate. I can not use the grid on the mat, messes me up, every time.
Good luck.