Feeling exhausted and tired is common with this type of works. Don't worry, as more dross and dirt is removed from your soul, you'll experience less and less collateral effects, just keep cleaning your entire soul.
We are all different individuals, some have little karma to remove (they already worked on it in past lives), some have a lot of karma and they experience very bad periods that cause deep transformations. It's perfectly normal.
Empowering your Solar Chakra is very helpful in countering low energy and tiredness.