Hi!

Your total of willpower and rage is always equal to your maximum willpower/rage. Energy spent is just transfered, there's no loss.
This means that if you spent willpower, you'll generate as many as rage.
And if you spent a certain amount of rage, you'll generate the same amount of willpower. So in your case you just have to use an attack skill (and so spend rage) to gain willpower and be able to cast spells if you got a catalyst/unique/Sarisel mode's weapon that allows it.

I've ran with Bulwark builds before, and I find that it helps to cut back on the stats you have that increase rage degen decay time, and watch out for skill mods that boost your rage, you may want to drop one or two. The key is to not always be hitting things with a build like that. take a breather from auto attack once in awhile and cast a few rage abilities like Amavabol mentions. also, if you decrease your rage degeneration decay timer your rage will start to fade faster. it's better to us stop the AA and use a few skills though.

right now there's only one spell that does sacred damage, so it feels very limiting, especially when it's a spell you cast one every 10sec or so. once the game is officially released, there will hopefully be more spells with that damage type, which means you will be able to build more of a hybrid kind of character using that shield.