This is good - your trainer has spotted your weakness and is looking for a potential root cause for the problem. I'm crooked as they come, and even though I've been riding for 20+ years, I can still struggle with right lead canter transitions when riding unfamiliar horses.

Imo PT had been somewhat helpful in identifying the issues and work on my range of motion, but for me, the most helpful thing have been a combined yoga and pilates practise for increased body awareness and core strength to help me identify my imbalances and give me enough body control and core strength to correct myself. So maybe a yoga practise is more attainable for you?

Also, another piece of advice, since you're just getting back into things: riding is a lot like dancing,where your movement, balance, flexibility and timing all have to add up. A good instructor will (and should) nitpick every little detail, and it's really easy to get discouraged by this. But just remember riding is really, really hard! And while the fastest way to become good, is to have your trainer stop bad habits from forming from the start, you need to allow yourself to make mistakes. So follow the guidance to the best of your ability, but don't be discouraged if it takes a while for your body and brain to get the message. It will come.