Cex obviously got a bunch of withdrawl requests, they batched them all together and sent it out...
You don't control any of that process as the wallet and keys are completely under their control... As are the way transactions are batched up and the fees used.
Talk to cex... I'm going to go ahead and guess they'll either ignore your request or give you some standard response about how they batch transactions to help minimise fees.
As you don't control that wallet, you can't stop them batching your transactions with others... Nor are you likely to be able to control the fees.
All you could do to unstick this is a CPFP, but on a ~10k byte transaction, the CPFP fees will be crippling!!?!