As for now, the only way that I know of is to buy the level again with experience. This would basically mean that after facing some difficulties during a few sessions my character became more resistant to stress (again). However, this is a very expensive thing.
Tremere (or Assamite Sorcerers) will inavitably lose Willpower during their unlife, it's not a question of "if", it's a question of "when". Didn't they find or develop another way of dealing with that risk?