I've the next redirect during my store:

Options +FollowSymLinks 
RewriteEngine on

RewriteCond %{HTTP_HOST} ^www.mydomain.net$ [NC]
RewriteRule ^(.*)$ http://www.mydomain.com/$1 [R=301,L]

However, I wish to disregard the redirection for
www.mydomain.net/admin

because its my after sales to handle my store and today I cant access my admin due to the URL redirection I set.

Any idea how to get this done?

Ought to be simple to simply reverse your line to some negation statement to ensure that it is applicable to everything that doesn't match the pathways you won't want to rewrite:

RewriteRule !^(admin/.*)$ http://www.mydomain.com/$1 [R=301,L]

or even more ideally just add another rewrite conditional to exclude the admin directory:

RewriteCond %{REQUEST_URI} !^/admin [NC]

Both of them should work however the second one looks better.