How to improve writing mathematics?

I want to highlight two tools for learning: imitation and practice.

Read a lot of mathematics. You will find that some texts are easier to follow than others. What makes you like a text? What texts do you like most? When you write, try to write as your favorite author would. If you keep on writing mathematics long enough, you will find your own voice, but imitation is a necessary first step.

Write a lot. You say you can write numbers and equations. What are they about? Explain. It doesn't have to be perfect, but explain it in your own words. Tell a story about your calculation. What would you say out loud to explain your work to a fellow student? Write that down. Try to make a habit out of making explained calculations that anyone could read.

Lastly, if you don't know how to write something, ask for help. Composing good mathematical prose is not trivial, and learning it is an important part of any degree in mathematics. You are entitled for help with it, not only with your calculations.

If I understand correctly, your problem is in writing relatively simple and short things. Writing and structuring a thesis, a paper, or other extended piece of work is a story I will exclude here.

At the early stages of my mathematical career, I found the following (little) book very useful.

Trzeciak, Jerzy, Writing mathematical papers in English. A practical guide, Zürich: European Mathematical Society Publishing House (ISBN 3-03719-014-0/pbk). 49 p. (2005). ZBL1077.00008.

Halmos, Paul R. "How to Write Mathematics." L'Enseignement Mathématique 16.2 (1970): 123-152. (PDF download.)

Idiosyncratic, but a classic. I especially enjoyed the section, "Think about the alphabet," and his discussion of "frozen" letters:

"many readers would feel offended if $n$ were used for a complex number, $\epsilon$ for a positive integer, and $z$ for a topological space. (A mathematician's nightmare is a sequence $n_\epsilon$ that tends to $0$ as $\epsilon$ becomes infinite.)"