You can't speed through the fields and pause at the same time. They contradict each other. You can speed through the fields and then stop to pause, but the way you've written it is that you're doing both things simultaneously. Also... how are you speeding? Are you driving a car? Running? Riding a bike?
Birds singing beautiful songs. The word "songs" here is a bit redundant - what else would they be singing if not songs?
A herd of deer of a hill... what? This feels too short. What are they doing?
I don't understand why your brain would be spinning at a million miles an hour in the first place as this doesn't seem like a particularly traumatic scene at all. Similarly to the "No panic. No shaking." Why would there be panic or shaking when sitting in a tranquil setting? Why mention the lack of these things?
Speaking of sitting in a tranquil setting, you never mentioned that you were sitting at all until the end. You were speeding and pausing, now suddenly you're sitting?
Final point - you start the entire piece with "Sun sets in the cool winter sky" but then end it with "I sit there till the sun sets".
So that just counters your first sentence, because you began with the sun having already set but clearly it had NOT already set. Which is it? When did this sun set?
I agree with the last poster about the overuse of short sentences as well.
I know that may seems like a lot of criticism, but there are also some nice descriptors. "When I look up, I witness a most breathtaking orange-red sky, mixing with the purple clouds" - this is nice, I can picture this scene.
The majority of the passage gives a lovely tranquil feeling.