How do I train a sluggish horse to go forward?

Hello! Start by having the horse checked over by a vet, and make sure the saddle and the rest of the tack sit correctly and are not pinching anywhere. Once all of that is sorted, I think the two of you should consider two things:

1️⃣ Horses are sensitive to pressure, so make sure the rider is not gripping with the legs, sitting heavily, or balancing on the reins. Horses want freedom from pressure, and when the pressure from the rider becomes overwhelming, they usually pick one of two responses:

❗️Running away from the pressure.

❗️Shutting down, going inward, and refusing to go forward.

2️⃣ Horses like being able to control their world, a world that we create for them. Left to themselves they would happily just graze with their friends, eat, rest, and breed. They have no idea what we have in mind for them to do with us, so we need to make our time together make sense to them.

❗️ Build the horse's understanding of your signals (the aids) gradually, over time.

❗️ Be consistent with the aids and always start with the lightest possible aid, the featherlight aid you dream of your horse answering, then reinforce until you get something close to the response you had in mind.

❗️A horse can feel a fly land on its coat, which means you can work with featherlight signals, as long as you give the horse time to connect them to the behaviour you want.

❗️ Horses hate surprises, so build your system for what the two of you will do in a way that lets the horse easily answer YES when you ask a question (give an aid).

Remember that changing a behaviour takes time, so be very patient!

Good luck 🙂