Ok here's my advice and I only read abit so not all details are clear to me.
1] pony have short backs so if you are to big for the pony and sit to far back your weight will be on the ponies kidneys, this hurts and sometimes they buck. This why most people don't allow people to ride double on there horses.
2]if the pony learns that I buck you off you go away then pony will never stop. Most pony's tend to be lazy by nature.
3] Teach pony that whoa means whoa and use it for mounting. Pony not allowed to move when you get on. Grab your left rein short and allow pony to turn in left circles so towards you until pony is prepared to stand nicely for you to get on. Don't panic or get frustrated just calmly allow pony to turn in circles until prepared to stand for you.
4]If bucking startes when you are on the pony remember pull pony's head up, harder to buck and a pony who is moving forward cannot buck. Another words don't allow interruption in the forward movement because then pony can really buck hard.
5] private email me if this doesn't help and tell me exactly what is going on because this is the number one thing that I deal with. Everyone wants to know how to stop there horse or pony from bucking.
To be safe always ride with a riding helmet, to protect your head just in case.