The Braisa says that one is not allowed to pasken on their own Bechor that it has a mum to allow himself to eat it. The gemara explains that we are speaking about a kohein who was given a bechor and qualifies as a יחיד מומחה who can normally pasken by himself, but we don't allow him to pasken on his own animal because we are afraid that he will be mo'reh heter for personal benefit.
Tosafos asks that based on this we should never permit anyone to pasken for themselves on any question of issur v'heter. Furthermore, the gemara in eiruvin says in that a talmid chacham can pasken for himself - how does this fit with our gemara?
The Rash in Negaim (cited by Gilyon HaShas) asks Tosafos question and says that it is dependent on whether it is אתחזק איסורא. When the item has a status of being forbidden and relies on the p'sak of the chacham to permit it, one cannot permit their own. But, when there is no default status of issur, one may pasken on their own. The Binas Adam (sha'ar issur v'heter 2) elaborates about this Rash and explains that when a question of issur v'heter would arise on the kashrus of an animal in the shechita process, that would qualify as a chezkas issur where one cannot pasken on their own. But, when it comes to checking the shechita knife on which there is no chezkas issur, one can check their own. It should follow from this that if the shechita knife would get a p'gam in it causing it to be invalid for shechita, the shochet shouldn't be able to check his own knife. However, the Lechem Chamudos (cited by binas adam) explains that anything which is one's control to fix, he is believed on even if it is אתחזק איסורא. Since the schochet can fix his knife by sharpening and removing the nicks, he is believed to say that it has been fixed.
The Binas Adam adds that we learn from our gemara that in a case which is אתחזק איסורא and not in one's own control to fix, even a talmid chacham isn't believed. This is clear from the case of Bechor where a יחיד מומחה isn't believed to pasken on his own.
Following this approach, the Chochmas Adam (109:6) paskens that although a husband may pasken on the bedika cloth of his wife (and she may pasken on her own), if a question develops about chatzitza while immersing in a mikva, the husband cannot pasken. The rationale for the distinction is that he can only pasken when it is not אתחזק איסורא. Once she is established as a Nida and the question is about her becoming tahora, it is אתחזק איסורא.