Short form: We all have our days and our challenges. Be kind to yourself.
Long form: I took advanced math (college level) in high school, have a college minor in statistics, worked with engineers and did payroll -- but I've lost most of my ability to do math in my head and some days I have terrible problems with just basic stuff that I used to be able to do with no problems. I don't know if it is being post-menopausal or depression/medication related or what, but I know it is a very real thing.
Bring it up with your doctor next time you go in. Some common medications (I'm looking at you statins like Lipitor) are known to give some people problems. Other people need some B vitamins or other supplements.
I now use calculators for things I used to figure in my head. I draw out things on graph paper. I have references on cutting techniques/dimensions. I've begun to use what I used to call "fabric wasteful" techniques, like when I have small triangles I used to cut them out individually, snub the pieces the right way and chain sew. Now for half-square triangles I do the "cut a square and draw/sew a line on the diagonal and trim off the excess". What I lose in fabric efficiency I make up in speed and precision. I also have vision issues and find in general it's just better for me to cut big and trim down.
And then we have some really great people on the board here to answer questions!