I like to use two rulers. I am right handed and I first line up the ruler to square off the fabric. I then put a ruler on top of the fabric measuring 3" (for example) I butt the ruler I am going to cut with against the first ruler If the first ruler is a short one, I slide it up to the top of the fabric and then to the bottom of the fabric checking to make sure the 3" is consistent. Then I cut my fabric. The bulk of my fabric is to the left side as I cut the pieces I need. I walk my hand up the ruler to hold it in place.
If you are short you might look into a portable table sold at Wally world. They are adjustable and a cutting mat would fit on this. Another thing you might try is using your ironing board at the adjusted height.