I have a defecation disorder, so I usually insert my finger into my anus to get out the excrement in order to be certain that there is nothing else to come out afterwards.

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.

It is impermissible for you to insert your finger into your anus as long as you perfectly cleanse the outer part. Doing that is an innovated act in religion. An-Nafrawi a Maaliki scholar, said:

"It is neither obligatory nor recommended for a person who makes Istinja’ to cleanse the internal parts of the two excretory organs. It is impermissible that a man inserts his finger into his anus and for a woman to insert her finger into her vagina. This constitutes undue hardship and is an innovated act in religion. This is considered sodomy on the part of men and lesbianism on the part of women."

Allaah Knows best.