Sometimes, writing down your thoughts, feelings, and experiences can be beneficial, but it can also make things worse. In general, it is likely to be detrimental if you try to understand yourself in isolation, but it can be helpful if it leads to greater understanding and behavior change in your interactions with others. When you're going through a difficult period, keeping a journal can cause your mind to spiral into a cycle of negativity. This doesn't mean it will happen to you automatically, however.
In fact, many people find great comfort when writing down negative thoughts in a diary, as it allows them to release these things from their minds. I started writing a journal about three months ago, and it was certainly cathartic for me. That said, I'm a veterinarian with PTSD who was recently diagnosed with autism, which explains a lot. My wife is also going through a health crisis.
All of these things started to pour out onto the pages of my diary, reliving old memories of times better left in the past, fears of the future - you get the idea. But I had made a promise to myself that I would write in my diary every day no matter what. At first it was painful, then it started to help a lot. I check my journal several times most days. So the short answer is yes, it might make you feel worse before it gets better, but I thought a perspective would help you more than just an answer.
We all have a past - some good, some bad - and we all need a safe place to confront it. That's what my diary is for me; I hope you find the peace you're looking for. This might be an oversimplified example, but it's possible that an obsessive habit of keeping a journal could cause you to constantly overthink your life. I would suggest giving the internal critic his own journal entry, listing what he criticizes you for being so negative - even though you had a good day - and then moving on. There is plenty of scientific evidence that proves that the practice of daily journaling is incredibly beneficial for your mood, stress levels, and physical and mental health. There's probably a reason why you've been drawn to keeping a diary and it can be helpful to keep that in mind as you go through the early stages. Keeping a journal seems to be less disruptive and more useful when what you write is already on the surface.
Another study published in the journal Stress Medicine found that keeping a diary can help boost your immune system. I would suggest doing a more rigorous journal - perhaps as a brain dump - when you find yourself getting angry about something. I have been furious and screamed in my diary for decades (although much less so in recent years), usually with positive results.