Lethargic behaviour is often from an underlying issue. Have a vet assess. Could need remedial shoes etc
Get saddle and back checked might be stiff somewhere if u can find someone who does shiatsui would strongly recommend it they massage the horse and treat for stiffness also high quality feed and pole work depending on your riding what I can’t comment on you might have to work on getting the horse working correctly through the back and lift his hind legs some horsss are way to heavy on the forehand and pretty much drag them selfs if the horse has a lack of muscle he/she might find it difficult also to who commended pinch boots please don’t irritate your horse and take the lazy route out for a quick fix and could just make everything a lot worse
She’s probably out of shape, or needs a feed change to promote energy.
You can also have a look at feed you're giving her, it might not provide her with enough energy and then if she was out of work it will take time to get her stamina back up so try some interval training to help it 😉😄
Try and get her to do loads of pole work risen trotting poles cavaletties canter poles also lunging and pole work it will give u an idea on what she’s doing with her feet but don’t over do it to the point where she bored!! Doing pole work will be good athletic work also making her pick her feet up while doing the work make it fun for her