I've kept a diary for well over 15 years now. I'd recommend:

1. Don't put pressure on yourself to write a lot. I often write a single sentence. If more is on your mind, write more, but most of my entries are a sentence or two, and that's fine, and still very meaningful to look back on. I can't stress the degree to which not putting pressure on myself to write a lot has allowed me to keep doing this, and yet some entries are very long, if I get caught up in it.

2. Build it into your routine. It fits into my morning stuff I'm doing like checking email (and I have a checklist for each morning).