Anxiety is the root issue I see here. when you become anxious like that, stress can turn into distress. Stress can be helpful because it can urge along what you gained in the studying process and jog your memory. Distress, however, can lead to harder to solve issues and locking up your mind.
If you can work on the anxiety, the handwriting will improve. Your hands get tired because your muscles are tense due to trying to speed up. The real issue is the nervousness. I'm sure (form your other posting history), you study a lot. That should bring you more confidence.
Perhaps to get help with anxiety you can try deep breathing as in yoga practices. Otherwise, if it were me I'd see a medical professional or psychologist. Sorry I can't suggest a better choice.