There’s a good chance she already knows, OP. As a 38 year old who has accepted (with the help of mental health professionals) that stuttering is something that *I* made into a big deal, and the majority of adults don’t really give a shit about (read: mature adults, and people who deserve your respect), it gets easier. Trust me. Good luck, but she’s probably going to say “I knew that already!”. Take it as an opportunity to see if she has questions about it. The more we talk about and acknowledge this shit (not trying to hide it), the less power it has over you.