Time. I like to groom my horses or used to before I had five.
Oddly the best way I found to finally "bond" with the hardest one, a feral pony. ..was grass. ;)
Treats didnt work, grooming didnt work, leading didnt work...they HELPED, she got a lot less afraid....but the biggest way she came to "like" me, and look for me was for me to hand pull the long bits of grass she couldnt reach and feed them to her. After that, she never let me put of hwr sight when I was at the barn.
The funny thing is, she was in no way starved, had plenty of grass hay, and is grained twice a day.