Nobody can predict how long an anxiety disorder will last. You only find out things like that at the end of your life, not the beginning. For some people an anxiety disorder will be a life-long thing. For others, it goes away as mysteriously as it began. And for others, it's episodic, coming and going. For still others, they get it, fix it in therapy or some other lifestyle change, and it's gone. Now, you may not actually have an anxiety disorder. Can't say based on what you're said. Being phobic about one thing, such as HIV or riding a roller coaster isn't necessarily a disorder. It's an isolated fear you either get over or don't. An anxiety disorder affects your life more generally than an individual phobia about something. What you have discovered so far is that you are prone to obsessing over not only HIV but apparently other health problems as well, or you wouldn't have been searching the internet for things that might be wrong with you. Good advice for all of us is, never search the internet based on symptoms you might have, it'll only drive you nuts. As you've found. The internet has good info and really bad info and lies on it, and you can't diagnose yourself on it. It's most useful when you have a health problem that has been diagnosed and you want to check the treatment the doctor is ordering up so you can see if your doctor knows what he or she is doing. Or if there are alternative treatments the doc didn't mention. There are some authoritative scholarly materials you can find there. But also a lot of chaff. If you're no longer feeling anxious and not having anxious thoughts, you don't have anxiety anymore. Relax and enjoy your life. If you're still having anxious thoughts and they are random and irrational and chronic, you have a problem that needs tending. Peace.
My experience having lived (survived) many years now, is I agree with the brain plasticity idea that the brain (mind) can slowly change, that the changes are real, and we can influence these changes with our thoughts (meditation, for example). I am certainly a different person than I was 10 years ago.
So no I do not believe one is condemned to live in anxiety forever. I believe this can change, though the change is slow (months, years).
I also believe things may not change if one does not put any effort or desire into changing. (Anxiety could last a lifetime if one didn't put any effort into effecting a change. It takes energy to rewire the brain for the better, so we do need to put that energy (mind thoughts) in to get the change we desire.
Fear is very common. Everyone understands. Abating fear, preventing it from running away, keeping it in check, is possible, and for my experience, having put in a fair amount of meditation has payed off. And now regular meditation, not much, just a little, helps keep me well.
So no i do not believe we are condemned to live forever unhappy and anxious. A lot changes over the years. It gets better.
There are a lot of things going on in the news right now that is frankly, anxiety provoking even if you don't have the disorder. How are you doing? How are you feeling right now?