Like the above posters have stated, you just need to keep it slow.
Maybe concentrate alot on ground work so you develop a strong relationship with the horse...the more bonding time you have with him, the more confidence you will have in him when you do get on him.
When you are up on him, concentrate alot on breathing...deep, regular breaths. Don't be too concerned with going fast, or doing alot undersaddle right now; just move at a pace that helps you feel most confident...the speed and gait changes can come later on.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."