How can I get over my anxiety or fear of driving on highways?

I would suggest driving on the highway at a time when it's mostly empty and when you have someone you trust in the passenger seat. That way if you get too nervous, they can talk you through it. I used to have this fear and what helped me get over it was exposing myself to the conditions I was afraid of, making it progressively harder once I was able to successfully complete the easier ones.

I used to get terrible anxiety when ever I traveled...I found the most effective method of managing my anxiety was controlling my breathing and to become very mindful of my breath. I would inhale and exhale slowly and deeply so that my mind would come under control and my body would also calm down and stop going into fight/flight mode. A lot of anxiety is related to our breath which causes a vicious circle, the more anxious we feel, the more shallow our breath comes which makes us feel more anxious and so on. By taking control of the breath we can take control of our mind.

I think the best approach would be to approach your anxiety or fear slowly. First think about what it is that makes you anxious or fearful about driving on the highway. Then slowly expose yourself to these elements in a safe way, possibly with a therapist, a close friend, or family member. Make sure that you are relaxed before exposing yourself to these things. While you are exposing yourself to driving on the highway, by doing something such as being in the passenger seat while someone else drives, do some relaxation exercises. Do not stop exposing yourself until you are able to stay calm while doing that thing. Eventually, try driving on the highway yourself (preferably with someone else) while relaxed. This is based on the cognitive behavioral therapeutic technique called systematic desensitization.