If you really want riding, powerstrike, shield AND throwing, then you'll have to sacrifice attribute points. 15/18 or 18/15 are doable (I got a HA similar, just IF instead of shield that).
Here's what I suggest if you want all of it:
Level: 30
Strength: 15
Agility: 18
Attr to skill: 2
Power Strike: 5
Shield: 6
Athletics: 4
Riding: 6
Horse Archery: 3
Power Throw: 5
Weapon Master: 6
Polearm: 104 (or 1h if you want)
Throwing: 130
This build here is a good HT build. Nice damage and the pole wpf let's you pick up a lance if you run out of ammo while mounted, fight with the melee mode of the throwing lance (really good melee weapon) or pickup a long axe/poleaxe if you get dismounted and survive it. Low athl, but that's what you have to sacrifice for horse throwing.
Level: 30
Strength: 21
Agility: 15
Skill to attr: 2
Power Strike: 7
Athletics: 3
Riding: 5
Horse Archery: 2
Power Throw: 7
Weapon Master: 5
Polearm: 85 (or 1h if want)
Throwing: 130