Bhí mé ag fiafraí díom féin (lit. I was asking myself)
Bhí mé ag déanamh iontais
Níl a fhios agam (of course this usually means “I don’t know”, but perfectly suffices in context to mean the same thing, i.e. “níl a fhios agam an mbeadh sé sin ceart go leor” = “I didn’t know/I was wondering that would be alright”).
Meas tú… – when posing as a rhetorical question to someone else, like “I wonder why she left..” when what you really mean is sort of implicitly like “why do you think she left?”
There are other ways as well.
As to the past tense of “ní fheadar” (which I usually just see as “n’fheadar”) I’m not sure if there is one. It’s a defective verb and I’ve personally never encountered it in any other form.