This is how I do it (my therapy is online consisting of chat + messages, but mainly chat twice a week).
I write everything I want to in a word document to release the tension and urge. Then I make it shorter and only leave the most important points. Imagine you're taking detailed notes from a textbook. I make sure not to write more than one page in a document, usually I keep it about half a page. I make sure to use paragraphs and proper formatting for easy reading.

I would like to message every single day, to chat with my therapist all the time. But I realise less is more and that sending too many or too long messages would cause my therapist to burn out and become fed up with me. So that would lower the quality of the therapy.

I message about 2-3 days a week in addition to chat sessions (when the therapy takes place). I use mails mainly to send the "homework" and comment on previous and future sessions.