(Concern) Stress from work, and gastritis

I am a station attendant and work directly with customers. As a result, I meet many customers, and I have often been emotionally hurt because of some people. Among those, there is a terrible memory that still occasionally comes to mind. In the fall of 2018, I was assaulted by a drunken customer while handling customer service. Despite my raising my voice and using profanity, when I refused to fulfill his demands, the drunken customer even slapped my cheek.

I had experienced symptoms of gastritis before and had undergone an endoscopy, but that day I felt extremely burning in my stomach and thought I might faint soon. I was so angry and excited that I thought that was the cause, but the next day there was no sign of improvement, so I went to the hospital. Since my stomach was already in bad shape and I was under stress, they said I had developed gastritis.

Even after this, when I see unreasonable customers going over the line, I often get headaches... Sometimes I feel nauseous and have a bitter taste in my mouth. I know I should not get stressed out to ease these symptoms, but since it's my job, what should I do?