Start simply with how you are currently feeling. Be as detailed as possible, and really get to the root of it. The reasons may surprise you.
I look for journaling challenges or devotionals. I follow a lot of Pastors and inspirational speakers who have books and content for me to read. I am currently doing a 21 day Journaling Challenge with Heather Lindsey. I signed up online through her website and she sends daily emails to guide each day. There is also a accountability group of other people doing the challenge also.
I use journal.isms. They are sets of cards designed to prompt reflection and introspective journaling. They are general or you can get specific cards based on what’s going on in your world: career change, breaks ups, “emergency” anxiety moments. They are awesome!!
The only way I was able to journal regularly was using one of those “One Line a day” book. When all you have are five lines to write on it doesn’t take much effort to fill them; also it is easy to write after, if day was missed.
I read a lot. I try to find questions and problems presented in the things I am reading and apply them to my situation. This provides insight and keeps journaling from being repetitive.