Is journaling actually useful or does it just feel productive?

It depends heavily on how you do it. Simply recording events has little effect. What has research support is expressive writing — writing about a difficult experience and, importantly, how you make sense of it, for fifteen or twenty minutes across a few days. Structured prompts also work: what is within my control here, what would I tell a friend in this position, what is the evidence for and against this thought. Pure venting without any reframing can entrench rumination rather than relieve it, which is why some people feel worse after journaling.