First off - congrats for pursuing your dreams! I know it sound cheesy, but for those of us who have the horse-crazy gene, (and it sounds like you do!) you can keep coming back to horses time and time again. LOTS of adult riders had to take some time off for school or family. It may not seem like it right now, but you are following the path lots of us had to take.
My childhood riding was also limited by lack of funds and unsupportive parents, so I was really excited to get back into riding as an adult. Like you, I at first found a position as a part-time 'working student" that while not BAD didn’t really set me up for success. I learned plenty about horsekeeping, rehab and ground work, but didn’t get many hours in the saddle. I don’t regret it, but it didn’t bring me closer to my goal of owning and showing.
You have been given lots of great advice already. I will repeat the obvious. This is an EXPENSIVE lifestyle. Trainers need to make money, and sometimes that brings out some really unscrupulous practices, like over-using lesson horses, or over booking group lessons so no one gets enough attention, or jacking up lease rates. Keep an eye out for bad behaviors like this. It will take some trial and error to find ‘your village’ in the horse world. HANG IN THERE.
As others have mentioned, shop for trainers by volunteering or hanging out at a local show - being a gate steward is great since you can hear just about everything all the rail birds are saying. There is one trainer I wont go to for numerous reasons, but one is that I overheard her really disparaging a jr. rider she was teaching to another adult. Really mean stuff about a kid whose parents were paying her! No thanks - I’ll avoid that toxicity.
Its going to take awhile for your body to catch up with your brain. I 100% feel you on this frustration. Try to make yourself as fit as possible out of the saddle since that will help, but sometimes you just have to acknowledge that you are starting over and you have to grind through those beginning phases all over again. A patient instructor and lunge lessons are excellent for this 
Don’t give up. Keep saving money. Keep trying different instructors. I’m sorry you had such a rough go with that bad trainer. The sad fact is that there are plenty of bad trainers out there. Don’t be afraid to ask lots of questions and screen them as you would any service provider. Don’t forget that you hold the cards here. Volunteer for your local eventing associations and increase your horse network. I’ve had lots of good opportunities come via word-of-mouth from acquaintances I had met by volunteering at shows.
If all else fails, CoTH is such a good resource. I’ve seen plenty of “looking for jumping instructor in X county” threads and people will respond with honest feedback and recommendations. Riding is hard and demanding. The vast majority of people here are really supportive and we want to see each other succeed - so don’t be afraid to ask for help 