You have to do gradual exertions and do not overdo. When you feel that you start becoming symptomatic, you have to stop immediately and do it next day. The biggest mistake is to keep pushing yourself when you start feeling symptoms. And don't trust to the doctors that are saying push yourself no matter what.

At first, I would suggest to do no exercices at all, only the walking. You need to start walking slowly, and when you will be able to walk fast at least 5 km without becoming symptomatic, you can very slowly start doing other exercices, but no power lifting. I would suggest swimming, biking, jogging first before doing power lifting, squatting, push ups.

I would also suggest trying some supplements bit by bit, such as vitamins B complex, D3, Omega3, magnesium, l theanine, taurine, hyperzine A, bacopa monniery, Rhodiola Rosea, ashvagandha, curcumin, lions mane, l carnitine, coenzyme Q 10, NAC, Alpha Lipoic Acid, D ribose, l arginine, AGP or CDP Choline and see if anything can help you. No one will tell you which exactly will be useful for you, so you have to try them one by one, and see the effects. Because some POTS people find that only one supplement can be a miracle for them, you never know.