Damian needed you alive in order to open a gateway into the Hall of Echoes (Patriarch already refusing to help him). I don't know about being a "wimpy coward", but he doesn't seem to have matured a great deal emotionally since he was a 15 year old infatuated with his first girlfriend.
Maybe Ygerna gained strength from being soul forged with Damian. Maybe she learned more from awakening the aspect of the Lord of Chaos in Damian than she did from the Black Ring and her necromancer father (who was relatively strong, but not enough to defy the Black Ring's orders).
The time spent in the hall of echoes may have helped as well; presumably if she could reach out enough to find a mind weak enough to be manipulated but strong enough to resurrect her, she may have started off learning from the minds of the recently deceased. That and she did have a lot of free time on her hands to devote to a single problem...
