It is normal not to get experience for low level opponents (see link below for details). There is no need to unlearn Wisdom; depending on where you explore first and whether you do any of the battlefield dungeons, there will be opponents you would not get much/any experience from in any case.

Act 2 - not getting exp in BF


By design you can not lower the difficulty level once you switch to hardcore. You can, however, hex edit a file in the save folder to change the difficulty (or if you have a recent save before you switched you could just reload).

Need to get out of Hardcore Difficulty


Check your messages (click 'My Stuff' in the top menu bar, then 'My Messages').