I’m incredibly intimidated by my boyfriend’s primary. To the point where I’m scared to kiss or even touch him around her because I’m worried I’ll step on her toes or upset her. She’s never reacted like that but I’m scared anyway. I know I need to communicate this to him and likely her, but I just don’t know how to bring it up… Any advice?
I think you’re off to a really good start here. You know this is important to communicate clearly, and you recognize that this feeling is coming from you, and not her behavior. That’s actually really key, and takes a lot of emotional insight. There’s a huge difference between “she makes me feel X” and “I feel X around her.” So kudos to you for avoiding unnecessary accusations and owning your feelings!
I would recommend just bringing this up with your boyfriend in a neutral setting - when you’re alone together and not dealing with anything else. Bring it up just like you’ve said it here - that you feel intimidated by her and you don’t clearly understand what the boundaries are when you three are together. You have a right to clearly stated expectations and boundaries, and a definition of what “stepping on toes” entails within the specific of this relationship.
Let him know what you need, as clearly as you can. It sounds like you need some guidance on how he and his primary want this arrangement to look, and some support as you navigate your way into it. If he reassures you by saying that his primary does like you and isn’t threatened by you, remember that he would know! If you’re comfortable with this, cultivating a friendship with her will help as well - when the three of you are together, strike up conversation, get to know her, and get better at reading signals and trusting that your presence is welcome.