Nothing can replace professional help. Even when it comes to journaling, they can have great exercises, tips, and ways of helping you reduce your anxiety, so that will always be my top advice. So if you can, I would suggest speaking to someone with authority, which I do not have, I only have my own personal experience to share.
Here I will write just what has helped me in the past overcome some of my anxiety, hoping this can help you too.
Writing about your anxiety may not always be an easy task, but if it is something that you are willing to try, it can actually have great positive effects.
If you are not sure where to begin when writing about your anxiety, try first take a few moments to write about what you are currently feeling, and what is currently going on in your mind.
This is just to get you into the mindset of writing, to allow your mind to ease up a little bit before getting into the heavier stuff.
After that, start writing down the feelings you have that you call anxiety. Try to write what it causes you to actually feel, and try to start identifying the cause or origin of these feelings.
Now, try to actually think about the matter that is causing you anxiety. Write it down however it comes to you. Don't try to write it from the beginning, or from the end, just write what comes to your mind, and from there, start exploring.
Try to explore why you feel anxiety about what ever it is that is causing it. If you don't know what is causing it, that is fine, just write down what you think it is. Explore different thoughts about it, where it came from, what you fear could happen, and what ever else comes to your mind.
It is important though to not linger on the anxiety and to not keep writing the same things down. This could cause you to just stay in the cycle of continually thinking about negative things that are causing the anxiety. Try to write things down with the intention of letting them go and moving on. Allowing the negative thoughts, worries, and fears to pass. Don't try and fight them either, just let them come in to your mind, and out on to the paper.
Again, I am not a professional, this is just what I have done in the past to help me overcome anxiety.