is exercise on the table?

Hi! I've got POTS, CFS, Fibromyalgia, Autoimmune Arthritis, and (not diagnosed) chronic migraines. While these conditions do absolutely impact my day-to-day life, I'm lucky enough that I don't need mobility aids and my medications are keeping the worst of my symptoms in check. This means that I can go out and socialise, or run errands, with only probably a 30% chance of a POTS episode, even if I'll pay for it for days afterwards due to my CFS. I'm 18F, if that matters.

With this in mind, is there any possible way for me to still exercise? I used to love hiking and rode horses for 6 years, but now I can barely handle a slight incline when walking before my heart rate drastically spikes. I'm a healthy weight, but would definitely classify myself as 'chubby' (5.0", 58kgs). I still walk indoors for maybe half an hour a day, but there's absolutely no chance I could handle going for an outdoor walk or anywhere where it isn't flat and air-conditioned. Without much variation to my diet, does anyone have tips for building muscle? I don't care as much about loosing weight, necessarily - just want to build up some muscle without throwing up or crashing for three days afterwards. Feel free to re-direct me somewhere else if this isn't the right place to ask!