I can never think of anything to write about unseen literature during exams- how can I improve?

No problem! Finding things to say is tricky, especially under test conditions. The first thing we need to do is go through the tick-list of what needs including- an introduction and conclusion, and roughly three paragraphs, each with a leading sentence and using the PEE system (all of which we can go through in detail). Structure is essential and can't be forgotten about, but once you have your structure sorted, you can start thinking about analysis and points.
If you always feel like the whole poem is an information overload, a really useful exercise is trying to write a practice answer on only one line of the poem, with one paragraph on structure, and two on features of language. This way, you force your brain to get creative with your points. For example, the line 'they drove past the cherry blossom tree, and through the gloomy gates' doesn't seem all that remarkable, but there's loads to say. Let's have our question be: 'How does the author present nature in this extract?'. For our structure point, we could say that the author's use of the comma, which physically separates the tree (nature) and the gates (man-made), is used to represent the divide between nature and the man-made world. For one of our language points, we could say that the juxtaposition of the harsh-sounding alliteration in 'gloomy gate' and the gentle sounds of 'blossom' highlight the author's belief that nature is 'good', while the man-made world is 'bad'. This might all feel like a leap, but being able to get really creative with just one line usually means you'll usually not be stuck in the future when you have a whole extract to play with. I hope this helped!