How can I level up more slowly?
If playing on the PC, the mod, SkyTweak can allow you to decrease (or increase) your experience gain rate.
For example, in this screenshot of the SkyTweak settings here, I am setting the One-Handed skill experience gain rate to half of what it is originally:
You might also want to see How to make Skyrim more challenging at higher character levels?
In Skyrim, character progression is solely based on levelling skills. Having said that, there are not many ways you can do to prevent levelling "too fast". However, doing the following should slow your progress:
Do not use Warrior, Mage, or Thief standing stone. These will increase the experience you gain from doing relevant thing. For example, if you have Warrior standing stone activated, you will level all your melee combat skill faster when performing those actions.
Do not pay NPC and learn any skills from them.
This might not be a real solution, but stick to one particular combat strategy will also help a little bit. For example, if you've been doing two-handed weapon so far, stick with it. Skills tend to level faster at low level, so by NOT performing those actions will prevent you from levelling fast.
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(Warning: You will lose all Stolen items doing this so be sure to be clean)
Goto Jail and serve your time.
To do this, you need to whack a guard, or steal something to the point where you get the option to goto jail. Accept, and do your time by sleeping in the provided bed. This resets all skills progress. (NOT THE LEVEL, JUST THE PROGRESS TO THE NEXT ONE)Rinse and repeat for loss of levels.