Journaling is a great practice to supplement therapy, but it is not a substitution to therapy. Many therapists suggest to their patients to start journaling if it is something that they believe they will benefit from. They also provide guidelines and further help with the practice, and even go over the notes during their next therapy session.
Journaling can be a powerful tool, and it can really help make a big difference, but it cannot be emphasized enough that journaling is not a substitute for therapy, but it can be an excellent supplement to it. If you are in therapy, consult with your therapist. Ask them what they think about it, and they will most likely be able to give you instructions or things to focus on while journaling.