The best thing to do is just be around them. Bring a chair to the pasture and just sit there. You don't have to chase them down and pet them. Just be there. The number one thing to horseback riding is bonding with the horse. Comfort when riding will come in time, but if you can't be on the ground with a horse, you can never be in the saddle. Erase your bad memories with good! Feed your horses treats, tell them secrets, sing to them. It sounds stupid, I know, but when you are 100% open with a horse, you gain this trust. It's weird but amazing.