what Zazen can do now

Zazen has had an uphill battle, but he has come miles from where he started. He is slowly building his trust of me, but sometimes we run into glitches. At this point I can lead, groom, saddle, pick up feet, trim, yield hindquarters and fore, all calmly and willingly. I thought things were going along fairly well, UNTIL, I asked him to take the next step, and ran into a stumbling block. He was doing so well that I thought it was time to finally get him under saddle; after all, I'm paying board on this boy! So, one day when he was doing particularly well in hand, I moved him over to a mounting block, and stood up on it. No problem. Lean over his back, little bit of a problem but he settled quickly. I made sure he saw me from the other side, and patted the saddle on the off side. So far so good. I wasn't planning on getting on him, because I was alone, and with his history that wouldn't be safe. Was just going to put weight in the stirrup- well, guess what happened?? Soon as he felt the weight in the stirrup, he froze. He was mentally back to a year ago- a complete wreck. So I backed off, went over some things he does well until he got his brain back together, and came up with a new gameplan.