Just be honest and professional. Looks like you already talked. Maintain that you were venting and the things you said may not represent represent how you truly feel. An honest apology will go a long way.

No place worth working for will fire you over one indiscretion. You will need to repair the relationship over time though.

On the other hand if you truly mean all those things you said and hate working there, it’s a good opportunity to start looking for a better place. No one want to be in a toxic work place, and no one wants to deal with an employee who clearly does not want to be there.