Keeping a portfolio places value on your work and observing your images over time is part of the therapeutic process. As you continue to build a portfolio, it's meaningful to look back on older images. What has changed? Is your style different? Your focus? Your voice? Are you more drawn to certain colors? Certain times of day? Certain subjects?
Additionally, over time your emotions will vary. Perhaps you started a therapeutic portfolio during a particularly emotional time in your life- those images may convey different emotions to you than images produced at a different time. Photographs have the power to freeze emotions so that we can study them and understand them in a meaningful way without interfering with the process.
A portfolio of any kind takes commitment. Like therapy, you must dedicate yourself to the process. Set aside time every day if possible, or at least a few times each week, to create images. Remember there are no expectations in therapeutic photography, and your portfolio does not have to ever be seen by anyone other than yourself. Let it be messy, let it be technically flawed, break all of the rules. Just make images, and let those images be your voice if only for the second it takes to press the shutter.