How can I cure my fear of the jump?

For the past several years I have wanted to go sky diving and/or bungee jumping but I am scared. My fear is not the falling (I am looking forward to that) but rather the jump or push out of the plane / off the bridge.

I know that’s what it is because I can’t jump off of anything much higher than a foot (even if I can see the ground I still have to find another way to slide off or climb down.

I do not know where this fear come from but I need to get over it.