DD2 has PoTs, started having obvious symptoms at 13, now 18 and at uni. She faints once a week on average, sadly doesn't get any warning, and as she can also faint while sitting, isn't allowed to drive
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In her case, lifestyle management helps a bit (drink lots of water, add lots of salt to food, regular bedtimes/getting up in the morning, minimising stress as much as possible). Exercise helps massively, and if you're in the position of not having become deconditioned, keep up exercise as much as possible - DD aims for at least 30 minutes of upright aerobic activity per week.
Medication helps, but it's very much trial and error, as what works for some, doesn't work for others. IME it very much helps to be seeing a PoTS specialist - DD has a "very much not advised" tablet regime, but one that works for her, as her consultant is happy for her to experiment with doseages. Other consultants are not as flexible.
Long term prognosis? Lots of people grow out of it in late teens/early 20s, especially if it started following illness. For the rest, it's a long term thing, that you can (pretty much) manage the symptoms for. DD2 gets DLA at the moment because of how much she's affected, but she's more or less managing at uni. I'm trying very hard not to think about life after uni, as variable medical conditions aren't especially attractive to employers, but we'll deal with that when it's time. In the meantime she has a big circle of very supportive friends, school, and now uni, make allowances where needed, and I'm there on the end of the phone several times a day.
A positive attitude (for both you and your DD) are vital IME - DD refused to let it beat her, so on very many occasions I'd drive her in to school, and not even have got back home before they were ringing to get me to collect her, but she was back again next day, determined that she was going to last out at least one whole lesson. Took a while to get there, and we've had far too many times when she's been lying on the floor sobbing about how rubbish her life is, but now, I think both she and I are in a position that we know as much as we can about how it affects her, what she can (and can't) do about it, and how to make sure she gets the support she's entitled to.
HTH a bit