When you read mathematical writing for your own understanding, sometimes it goes smoothly and sometimes not. When it doesn't, that may be because the mathematical ideas are inherently difficult and complicated; but sometimes it's because the notation, the sentences, or the argument aren't as well expressed as they could be. In the latter case, try rewriting the awkward section in a better way so that the meaning is clear.
After you have practiced this habit for a while, try transferring it to previous pieces of your own writing. Finally, after leaving it for a few days, critically revise your new writing.
Additionally, I suggest that you read the works of great mathematicians who strike you as writing particularly well. Examine the details of how they write, from the smallest scale (e.g. how they use subscripts and superscripts, and how they punctuate) to the largest—for example, how they cross-reference, so that the reader who has lost the thread can easily pick it up again, and how they use repetition and alternative description to reinforce the reader's understanding of key ideas.