My metamour prefers to think of me as an elderly pet and ignore the fact he's in a serious relationship which is a big commitment in his life. Separate relationships absolutely work. However, her extreme level of discomfort and general lack of consideration for his other commitments is not compatible with the level of relationship she obviously wants with him. Her approach means he will need to pick between us more often in the future and it means we never discuss things as a three. She's the last to know and plans are made which indirectly affect her but she has no real say in.
Unfortunately, neither her or my partner understand why this isn't sustainable but they'll live and learn.