Hi,
Here's a very general comment on verbal prefixes.
The prefix 'mis' has the idea of 'do something incorrectly'.
eg count/miscount, label/mislabel, quote/misquote
So 'I mistrust him' means 'I feel it's incorrect/wrong to trust him'.
The prefix 'dis' has the idea of 'do the opposite'.
eg regard/disregard, qualify/disqualify, lo