I have to work in excel everyday doing some pretty complex analytical work. There's probably a few things going on but if you're having trouble thinking through some simple math problems you probably aren't ready for the computer yet. I made the mistake of trying to push through this and ended up with some new and more serious symptoms. Do you have trouble focusing and tracking things? If so yo should be working on extraocular movements like saccades, anti-saccades, and smooth pursuits. If you aren't familiar with these look them up!