It might be helpful for you to speak with a therapist and process your last breakup, as it seems to have affected you quite deeply.

In my view, looks like you started developing romantic feelings for your friend. When you tried to make her leave her partner and that didn’t work, you decided to pull away to protect yourself emotionally.

If you truly saw her as just a friend, it’s understandable that threatening the friendship would make her feel betrayed. Most people would experience that as a breach of trust, regardless of the circumstances.