Black beauty!

The issue you are speaking of is called barn sour. This issue is quite common for horses.

The problem is that the horse feels safe near or in the barn. When they leave the confines of the "safe zone" the horse starts to experience anxiety.

There are 2 paths that you should take to start solving this problem.

Start building a strong relationship with the horse. Make the horse feel more comfortable and safe aroundd the people who ride it. This will help the horses anxiety when leaving the "safe zone".
Keep the horse's brain active, so it does not want to go to the "safe zone" this can be done by working with the horse away from the barn. You may want to take small steps and start this work close to the barn, then move further away. Also keep things freshh, for example if you usually head for the barn after a ride, try to go somewhere else and maybe walk the horse for a while, before heading to the barn. You can take a look online for ideas to keep your horse's brain active.

I hope this helps and good luck,
@tincanjunction