I have so many doubts concerning what is Najis and clean and now I have doubts on whether I purified something properly. Can I forget about these doubts?

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There is a fixed rule you please always keep in your mind then inshallah you will not have any doubt regarding Taharat and Najasat concerning anything.
Everything is Pak unless a person is sure(100%)about its Najasat. Also if you are sure about being Najis anything it will not be considered Pak unless you will not become sure about its Taharat.
So don’t get doubts unnecessarily if have made it Pak followed by Shariya rule.