This is so sad and good! I loved it

This is an interesting concept and I like the fact that you stayed with what has been established in the MCU (Bucky being Steve's friend and Sharon the girlfriend. Not a huge fan of "stucky" or erasing Sharon Carter simply due to a connection with Steve).

The dialogue is fun to read and not too out of character. As a piece of advice, from a reader's perspective, I would prefer to see more description so that envisioning the scene mentally is easier and therefore engrosses me more in the story.

The short quick-paced paragraph is a clever way of structuring Steve's current mindset, but at times I felt that because of the lack of description they were unnecessary to the main idea of the chapter. And I think that more description and thoughts from Sharon's side would make the writing flow better because her characterization seemed quite abrupt.

I hope this helped because one can't improve their writing without (constructive) criticism. For a first attempt, it's not too bad and there is certainly potential for this to become interesting with a few tweaks and clear description. I don't know if you will continue this, but I'd wouldn't mind to see more of it!

If you'd like to know, that username is my actual username you can search to find me. I would be happy to help or collaborate one day :)