From beta reading to editing

Lately I've been quite busy with beta-reading and reading for reviews. It's amazing how different people use language, how they diversify the syntax - or don't, how sometimes dialogues seem to be taken from real life and other times they are so constrained and forced.

The beta reading I mentioned, the one that is still continuing, led to copy-editing. This week he wants to send the first three chapters to a publisher.

I still can't understand how it's possible that I do this job in English. I really enjoy it, but I'm full of doubts. How can I ever make suggestions about grammar or syntax to a native speaker? How dare I?

Sure enough, I can locate where it should be "it's" instead of "its" or when the author jumps randomly through the tenses. But my vocabulary is limited, when I write, I'm never sure that I get it right.

I just can't read anything without noticing mistakes. And, to find them in published books is disturbing, especially when the acknowledgements mention the fantastic proofreader. I know I can do better. So, why not?

Some authors really seem to appreciate my corrections and suggestions, my thoughts about the story line, the characters or specific scenes. What a boost for my ego!
Am I helping the authors or myself? Ideally both.