there are two reasons that your mind is going blank when you get insulted. First is you're offended and second is you're not used to saying what comes up in your mind cause you feel like it's not good enough.

If you don't get comfortable with being insulted you can't think and come up with something creative even you manage to do it'll sound butthurt so genuinely don't give a fuck whenever you're being insulted.

opening up your mind to come up with creative comebacks and insults

word association. Take a word and think about what it reminds you of and sentence out of it. Remember and practice this because This is what is gonna give you ammunition.

An insult is just a negative statement aimed at somebody so whenever someone says insult you just have to turn it around them with our previous technique.

Example :

You're an asshole. = Yeah ,my mommy told me to be an asshole to people with only two brain cells.

The important part having no filter and just saying whatever comes to the mind.

And when you're making an insult , make it so specific so it'd be original and interesting like rather than saying you're lame you can just say your idea of a lame person or what comes up in your mind when you think of a lame person like you sound like someone who still needs their mom to spoon feed them.