First comes diagnosis...is it that your cuts are off, or is it that your seam allowance is off?
If you don't have one yet, I highly recommend a quilting foot with a guide. The foot itself has this little "blade" (not sharp, just a guide) that you can butt your fabric up against - that can
really be helpful in getting good consistent seam allowances. You can buy these on Amazon for about 8-12 dollars, or less if you hunt around.
Test your seam allowance - cut yourself three blocks of fabric that are 2-1/2 inch square and sew them together (after making sure they're cut accurately). You should end up with a strip of fabric 6-1/2 inches long. Your center square should have exactly 2 inches of fabric showing on the face. If not, your seam allowances are off. You may need to adjust your needle a little bit, even with the quarter-inch guide foot.
If your cuts are off, it could be a few things. Hardware - are you using a rotary cutter and quilting rulers you can see through easily? Those aren't required but they definitely make the job easier. Make sure you have good lighting and all that jazz so you can really see what you are doing. Make sure your ruler isn't slipping - you can put a dab of silicone on the backs if you have some laying around, just a smudge of silicone in a few strategic places will help. Or there are a myriad of products sold for this issue - I'm fond of little silicone sticky rings.
Second comes forgiveness. My first few quilts were all OVER the place and seams lined up by accident more than anything else! This is a hobby that takes a bit of practice and you should not beat yourself up for not being an amazing quilter without having had that practice yet. We
all make a few wonky quilts at the start. (Some of us never stop! LOL) Allow yourself to be a beginner.
