DD2 has this, currently her worst symptom is fatigue (she's 15 and fell asleep at 8pm tonight
).
Lots of salt, lots of water, constantly fidget, pump your feet up and down, jiggle about when standing, plan your bending down (timing, position - is it safe if I faint).
Brain fog is horrid - write notes, lists, and be kind to yourself about forgetting everything
Do you faint? Or just feel dizzy? Squat down if you feel faint, or lie on your back with your legs in the air (your body is letting blood pool in your feet and legs rather than it circulating, which is why the heart races, to get the blood moving, if you can get the blood away from the extremities, things are easier)
Exercise -dd is supposed to do 30 minutes of " hot and sweaty" upright exercise every day (doesn't happen always, but it's the target).
We're currently experimenting with a low GI diet, probably going to do more low carb, which seems to help a bit (tho possibly it's just cutting out the rubbish
)
Look at STARS and POTS UK (on phone so links are tricky) for more info (STARS has a patient day on 20th October with one of the top US experts on PoTS speaking), also google dysautonomia which is what it's called in the states.
There is medication which can help, fludrocortisone increases blood volume, Midodrine tightens up the veins, betablockers regulate the heart rate, Ivabradine does similar without affecting blood pressure. People are on different combinations and doseages as everyone is different , dd takes fludrocortisone and is experimenting with amounts and timings of Midodrine, betablockers worked brilliantly, but her hair was falling out, so we ditched those 
Are you seeing a specialist? There are more specialists for adults around, and thankfully a couple of paediatricians are appearing.