The phrase my writing refers to what I have written. It could mean my handwriting, or essays, or papers that I've written. It refers to your writing in general, whereas sentences is rather specific.
Ex: Here are the sentence I wrote.
Ex: Here is the writing I wrote. <Meaning, sentences>
Ex: Here is my writing.
Note, writing doesn't take -s (*writings), and it agrees with a singular verb (My writing is terrible!).
All the best.
- For Teachers