A shy protagonist often feels like she's doomed to watch from the sidelines, but I've seen a couple of patterns that make it feel less like magic and more like a believable push. It's rarely a sudden transformation. Instead, it's a series of tiny, forced steps—being thrown into a group project at work where she has to speak, or accidentally getting paired with the extremely outgoing love interest who just... doesn't let her fade away. That external nudge is crucial.
What sells it for me is when her internal monologue stays anxious and real, even while her actions change. She might still be internally panicking while agreeing to a coffee date. The key is having the love interest notice her quiet strengths, like her observational skills or kindness, and valuing those instead of trying to turn her into someone else. The 'overcoming' feels like an expansion of herself, not an erasure.
I think the most realistic versions show her gaining confidence in one specific area tied to the relationship first, like trusting that one person, before it slowly bleeds into other social situations. It's a quiet arc, and honestly, sometimes the appeal is that she doesn't fully 'overcome' it, but finds someone who makes her world feel safe enough to be a little bigger.