Your working memory capacity can be overloaded in three ways, making you feel mentally drained:

• New routines can prevent you from the ability to do things on auto-pilot and will instead draw on your limited working memory capacity.
• Anxiety also reduces your working memory capacity, making it more challenging to work through any mental problem that needs problem-solving.
• Distractions that are not directly relevant to your tasks can further increase the demand on your working memory capacity.