Creating a connection with a horse
I can relate- my newest horse was indifferent to humans, not afraid, just seemed to view us humans as an uninteresting part of the landscape.
Two specific modalities made all the difference in the world- a complete 180- she's friendly as a dog now, while remaining respectful.
First, I read Gary Wilkes and Alexandra Kurland on how to clicker train, and I started clicker training her. She took to clicker training like a duck to water, and she LOVES IT! . Even without the treats, being clicked for desired behavior gets her very happy. It transfers to all ground work and to riding, and of course I phase out the clicker into voice, touch, and hand signal commands as soon as appropriate, while still using the clicker to teach new commands, reinforce desired behavior, and to "capture" desired behavior.
Secondly, I got out my Jack Meagher Stress Point therapy book, and started working on her. She is now even more lovey-dovey, while still remaining respectful- I insist on this for safety. She comes up to me now of her own volition, as well as coming when called (thanks to clicker training) from voice and hand signals.
I recommend first the clicker training, then the Jack Meagher body work.
For many years I have used and highly recommend Linda Tellington Jones methods too.
Also I have more many years used aromatherapy and Bach flower remedies.