I have eosinophilic esophagitis, which is a rare autoimmune condition. I am both lucky and unlucky, in that most sufferers discover they have this later in its progression, when they have difficulty swallowing, but I discovered mine early due to near-constant and severe left side chest pain. This is an unusual presentation, but it goes away when we get my EoE into remission and comes back when I relapse, so we're pretty sure it's the cause.
I have come in and out of remission over the years. Unfortunately, my case has been quite difficult to keep in remission, so I live with left-side chest pain most days. There are a wide variety of other exciting symptoms.
Before I got sick, I was a healthy person. I worked out five days a week. I traveled internationally. I ate widely. These have all become much, much harder.
So what I would say to people who are not chronically ill: it can happen to anyone. It can happen any time. All that stuff you think happens to other people, it can happen to you. I literally woke up one day in pain, and that was it. That was the end of the good part of my life. One day I was fine, the next day I was permanently not fine. I was 27. I worked very hard through my 20s thinking I had time to enjoy life. Don't put off living. This can happen to you.
On the whole, as others have said: one day at a time. You just keep going. You will not believe how much suffering you will be able to put up with and keep going. On bad days, I tell myself that I can still go for a walk. I can still watch a sunset. I can still pet a dog. Get a therapist. Talk to the therapist.
Other things that have helped me: extremely careful dietary control. Reminding myself of the hundreds of extremely more horrible diseases I don't have. A care team of doctors who I trust, having replaced the doctors I didn't trust (for instance, the first doctor I saw, who told me it was all in my head).
Chronic illness is a very hard experience. I do not recommend it.