Can you give yourself a learning disorder if you are constantly stressing over the possibility of having one?

No. One cannot "catch" a learning disorder. It is possible that you are worried that you have one because you are noticing that certain types of learning are harder than others. But, learning disorders are hard-wired problems of how the brain processes information. You cannot worry yourself into creating that kind of problem.
Depends / definition. An individual can not exceed there abilities but can often do less. Consider a runner with the flu who runs a poor time. We all perform our best when:1.Well rested 2.Not hungry 3.Not ill 4. Are in a comfortable setting. Change any feature and we do less than our best. Obsessing about any issue will deminish output. True learning disabilities imply underperformance under the best of conditions.