When you have a horse who always moves away when you are trying to mount him, there could be 4 reasons for this:

1. The horse probaly isn't ridden enough - this can be solved with consistant and frequent work outs
2. The horse could have too much energy from excessive feed, making him a bit hot tempered
3. You could be putting too much of your foot in the stirrup and as a result end up kicking the horse in the barrel – that would make the horse try to move away
4. Could be that you are leaning back when trying to mount, in effect pulling the horse towards you which would cause the horse to compensate by moving away

Here's a tip to try: when you mount, make sure to shorten the rein on the side you are mounting, which will tip the horse's head to that particular side. If the horse tries to move off while you mount, you can easily pull him into a circle. Just make sure you aren't pulling on the horse's head as you mount as he will move his hindquarters away.