My Skyrim horse can only gallop (sprint) for a few seconds, same as me. Why is this

The creation kit has now been released and can be found on Steam.

When we created Skyrim, we set out to make the largest, most engrossing role-playing game we had ever attempted. Now it’s about to get a whole lot bigger, thanks to our amazing modding community.

Mod-makers have a long history with Bethesda Game Studios, bringing to life new quests, locations and characters, as well as making changes to game from subtle tweaks to full-blown overhauls. With the Creation Kit, the same tool we used to create Skyrim, there’s almost no limit to what can be accomplished – and we’re putting that power in your hands. Best of all, access to the Skyrim Workshop and Creation Kit is free for anybody with a Steam account and a copy of Skyrim.

With the Skyrim Workshop, finding and installing your favorite mods is easier than ever before. Mod-makers will also enjoy a streamlined process for uploading mods directly to Steam.

Here is a link to the steam workship for Skyrim.

Source: Creation Kit Wiki

Well, lacking a precise formula (even though there certainly is one, the problem can be nailed down mathematically since the only variables are the weight of items, the value of items, and the total amount that you can carry), all I can do is give you some tips:

• If you don't want to invest perk points in armor trees, get light armor. In the end game, light armor will give you the same protection as heavy armor does, but it is... well.. lighter. Source: http://elderscrolls.wikia.com/wiki/Category:Skyrim:_Armor
• Get rid of dragon bones! Although selling them might be a bad idea, since you can craft dragon bone armor if you get deep into the smithing skill tree.
• Stay away from 2H weapons. They are among the most heavy objects in the game, while usually not having a very high value.
• Store your books somewhere! At least I picked up every book that I could find, promising myself to read them one day. But carrying around 50 or so books (there are a lot in Skyrim!) takes up at least a good fifth or your carrying capacity - for little to no value. Get a house and put them in a box!
• Search for gems and soulstones! They weigh almost nothing and are usually decently valuable.
• The same goes for enchanted necklaces, rings, and robes. All of them are ridiculously light compared to their value.

These are the ones that come to mind right now.