"Does the disease eventually just burn itself out, or do some people just continue with it forever?"
The answer to that is: yes. Sorry, I know that isn't particularly helpful but it is now being recognised by rheumatologists that PMR is probably not a simple single disease - it is very likely heterogeneous and that means that not only are there differing causes but also different responses to treatments. There are forms that don't respond to pred or respond less well and there are forms that may be helped by DMARDs.
It is held to be an autoimmune disorder that causes your immune system to attack your body by mistake - that is what leads to the inflammation, pain and stiffness. Pred usually damps it down and allows a decent quality of life until it burns out - according to the rheumies. But the propensity to develop it remains - you can have second episodes. Or your first one lasts a long time. At a support group meeting recently I was sitting with 2 other ladies: one has had PMR for 15 years, one for 13 and I have had it over 12 years. None of us has had a period where we could get off pred. The 15-year lady is actually on pred for life because her adrenal glands are no longer functional. She has been on 5mg, it is being considered she may need 7.5mg. Last year I got below 5mg - only to experience profound fatigue (also adrenals????) and went back to 5mg where I felt really well, 4mg was OK but not as good. In January I had a flare that definitely appeared to be affecting my chest arteries - back to 15mg and now down to 10mg. The other lady struggles at below 15mg. All of us are of the opinion that we have to live - I'm only in my mid-60s, normally I'd still be working. No doubt I could get off pred, or at least to a lower dose, more quickly but I have no desire to go through the early part of the year again - since it was sorted at a high enough dose I've been able to achieve a lot. The only problem I'm aware of at 15mg is weight gain if I'm not religious about avoiding carbs - over the last 4 months I've been travelling a lot with holidays and conferences and family visits and haven't been able to choose what I eat. Result: 6 lbs weight gain! Home now - back to my sort of food!
You don't "contract" PMR, it develops - why no-one knows, but something finally forms the last straw for an overstressed immune system and it goes haywire. The triggers are thought to be wide ranging, environmental, chemical, stress of all sorts and there is a small genetic component: Scandinavian genes figure highly (those Vikings got all over the place) but it is rarely found in people of African or Asian heritage - rare doesn't mean never though, the rates are just far smaller. That could also be because it isn't recognised, as it very often isn't recognised in under 50s because many doctors are convinced it is a disease of the elderly, if you don't look, you don't find.
If you are looking after elderly parents - I'm not surprised you are always tired. It is quite obvious from the forums that people who have to work, or its equivalent in caring, need more pred to be able to manage. You can manage a lot of PMR with lifestyle changes and that usually allows you to get away with a bit lower dose but if that isn't possible you need a bit more pred.
Did you have any signs of the flare at 5mg in advance? To a great extent, the most common cause of flares is reducing too far or too fast. If you recognise niggles don't deny them - stop reducing and return to the last dose at which you felt good. That may be enough. It is also a shame the doctor put you right back to the start - that isn't always necessary and just adding 5mg to the dose at which the flare occurs often is enough and means the new reduction is shorter. When you get to the region of the previous flare - slow down and be really careful and watchful. You are never reducing relentlessly to zero - you are looking for the dose that manages the symptoms as well as the starting dose did. It doesn't mean you won't ever get off pred, just not yet. Rest at the lowest dose you find works - and not one that is just too low, there is no virtue in that, the residual inflammation will build up again, like a dripping tap fills a bucket, and you will have a flare sooner or later. Every few months try another 1/2mg, preferably using the slow reduction - if it works, rest there for a month or two and then try again. Below 7mg your body has to start to make cortisol again - it takes time, a staged return to work so to speak. The slower you go the easier your body will find it. Below 7mg is also a low dose, the side effects are less, and most doctors stop panicking at that level. My doctor wanted me back at 10mg by the summer - I've done that. He'd really like me at 8mg - and I'll work on that in the autumn. I'd like to be back at 5mg - but if it isn't going to happen that's fine too.
So - does that answer your questions? You WILL probably get off pred eventually - 75% of people do in up to 6 years or so. The German rheumatology textbook says 5 years or so. It is only the English-speaking community who are so fixated on the 2 year bit. I wish I understood why - I know some say it because they don't want to scare/upset patients. But it is even worse when after a couple of years they are still in the throes of PMR - they think they are to blame, that they have done something wrong and their doctors also often make them feel wrong. Then the forums have the job of explaining that the starting coordinates were wrong - that's why you are still in the middle of a field instead of on the motorway!