When that "Spirit Energy" meter appeared on the top left corner of my screen, the game didn't explain it at all and thus I didn't pay much attention to it.

But now I realize that the value of the meter drops slowly overtime and largely after I rest. And because of that, I suffer from Ability Scores decrease and could eventually die.

What's the way to deal with it? I play a Wizard. How on earth can I regain my spells if I can't rest??

And now my Spirit Energy has dropped to 30.

1 . The information is in the feats that you PC now have (At the top of the list), basically you has 4 new feats: Spirit Energy (That give the general info), surpress, devour spirit and another I don't remember but something tell me you need it right now (or load a previous save) as will restore you spirit meter by sacrificing XP.

2 - Basically, you need to suppress near to spirit (Tethors, undeads, actually the "Detect Spirit" feat of Gann give more info) or devour spirits, however the last increases your hungry which in turn make you drop the spirit meter faster and the former... well without abusing of the maths of the game only give you a little extra time (That is why everyone in the game know you will eventually die of the curse).

3 - Special conversation will recharge all your meter and probably give you new abilities which can be focus to suppress efficiently (or free undeads) or devour anything as living creatures souls or healing the party (And reducing the craving).

4 - Even the evil guy will probably need to suppress or don't sleep too much (Anyway, a epic wizard can do full missions without sleep)

Okku and One of the Many are spirits ,so are counted for the suppress feat, also the elemental summons and undeads. Not all the undead are spirit however (Again, if Gann don't detect them, they are not spirits)

@Belsirk,

Thanks. I'm in the first town, and there is now a spirit animal follows me and makes noises. And when that spirit animal appeared, Gann did say something like: "Someone needs to stop this thing......" So should I use Surpress near that animal spirit? If so, how much will my Spirit Energy increases?

I found a cycle at the following page (the 8th post) to replenish Spirit Energy to 100%. This way is easier, but a bit tiresome.
www.gamefaqs.com/boards/939027-neverwinter-nights-2-mask-of-the-betrayer/40775737#8

Is there a better way?

Yes, that is how you can exploit the maths in the expansion.
As you progress in the game you will get new abilities for your hunger, which will help you with the hunger and/or craving. But that is how you are planning to RP your wizard.

Thank you very much.

If it gets annoying, it can be turned off with the console; posts #3 and #4 in http://forum.bioware.com/topic/230099-anyway-you-can-turn-off-spirit-eating-as-i-keep-running-out-of-juice/

Thanks. With the cycle, it is no longer a problem.

Sadly, that forum no longer exists. Can you illustrate the process or provide another link?

The Wayback Machine has it archived: https://web.archive.org/web/20151030003455/http://forum.bioware.com/topic/230099-anyway-you-can-turn-off-spirit-eating-as-i-keep-running-out-of-juice/