I'd say you're doing extremely well.
Don't be afraid to keep asking for the reassurances that you need. If they're really interested in making this work, they will go at your pace. They've had a whole year to lose your trust; now they need to put in the work that should've been done from the start and help you through your negative feelings.
Negative emotions are perfectly normal, especially given the way you've come to poly. When they happen, acknowledge them, deal with them as best as possible, and try not to worry you're doing something "wrong" just because you're having those feelings. You're not. I don't think anyone around here hasn't had negative emotions in their poly experiences. They're a part of life just like the good ones.
Also, technically, she's
entering the game late, assuming you've been with your husband longer than the year they've been together.