All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
Scholars underlined that non-liability is the basic principle; the person is not liable for anything unless it is proven with certainty or prevalent likelihood that he is responsible for it. Al-Juwayni said, "Non-liability (i.e. freedom from obligations and liability) is the basic principle in all matters that are doubted. This applies to the rights of individuals pertaining to properties: transactions and the private and public rights." [Ghiyaath Al-Umam]
It is feared that what you mentioned is a sort of unnecessarily overburdening yourself because you suffer from Waswaas (obsessive satanic whisperings).
Generally speaking, there is no way to know whether you caused the patient's death or not except by consulting skillful specialists. They are the ones who can decide the cause of death and whether you were responsible for it or not.
Since it was not proven that you are responsible for this death, you should act upon the basic principle; you are not liable for anything, and you are not held responsible for anything.
It should be noted that the person bears no sin for the actions done out of mistake or forgetfulness. Allaah, The Exalted, says (what means) about the supplications of the believers: {Our Lord, do not impose blame upon us if we have forgotten or erred.} [Quran 2:286] The relevant hadeeth says that Allaah answers their supplication: "He (Allaah) said, 'I indeed did.'" [Muslim] However, the person may be still liable for compensation.
We have previously underlined the doctor's liability for medical mistakes in fatwa 87930.
Allaah knows best.