To answer the OP’s question:
I’m currently going through RCIA, and as a cradle Protestant I still have several difficulties concerning these doctrines. However, I am largely convinced, for various reasons, that the Catholic Church is indeed what she claims to be. If that’s the case, then it follows that the Church has the competence to teach infallibly on precisely these sorts of subjects.
I would be lying if I said that this posed no further difficulties. Even though the above logic implies that the Church’s Marian teachings are correct, it doesn’t necessarily make it easier to internalize them, or to quash numerous inner objections that still seem sound, at the moment. Whatever trouble I might have in understanding or seeing the truth of certain teachings, though, I can still obey and assent to what reason tells me is the valid authority on the subject. I can refrain from giving scandal by airing my difficulties in the wrong company. And I can hope that with prayer, time, and grace, obedience might blossom into true, heartfelt understanding.
This might seem like weak and faithless stuff, and maybe it is, God help me. But I hope it’s not being stiff-necked, so much as an acknowledgment that conforming one’s mind and heart to the shapes they ought to have can be a long-term project, depending on where we start from.