Whatever you do, whatever they do to you, don't quit right now. Make sure you work the overtime. Check and recheck the schedules so you don't miss being scheduled. Do your job to the best of your abilities. If you are written up for a performance issue, tell them you are doing the best you can. Do not be beligerant, do not be pushy, do not be touchy. Your goal is to get out with a severance package and unemployment insurance until you get another job. You've got about a month to put up with this, hang in there!
Actually, legally in your state, the employer can pretty much fire you for any reason they come up with, or no reason,just because they feel like it.
If you voluntarily quit, even if the boss is yelling at you and telling you to quit or they will fire you and ruin your references forever, pleasantly tell them they'll have to do what they feel is the right thing to do. Make them fire you, even if it is for a trumped up reason. If they are stupid enough to ask you to work unpaid overtime, call the state wage and hour office and file a complaint. But do not quit. Do not give them a legitimate reason to terminate you. Do not be late. Do not be absent from work without a medical excuse. Be cooperative, do your best, be a real model employee. It will pay in the long run.
I am not a bit sure they have to give you severance, regardless, unless it is in some sort of contractual agreement you have with them, someone else here can be more helpful about this, but if you quit before the job ends
instead of being let go through no fault of your own, you do not get unemployment insurance. And if they terminate you for a reasonable valid misconduct reason, you would not get unemployment insurance. So make sure you do not quit and that they do not have a valid misconduct reason to terminate you. If you are doing your best on your job, even though they say that is not good enough, it is not considered misconduct.