I'm totally, completely burned out. My nearly 90 Mom is sundowning. The constant screaming, yelling and criticizing are unbearable. Advice?

Your mom’s care has become more than you can handle. Not your fault as you’ve done your best. Please do your best to protect your own health. Call 911 next time things get out of control. Tell the hospital staff that you no longer can provide in home care. This doesn’t take away your relationship or role as caregiver, but it’ll change in a way that makes life manageable again. I wish you both peace

You may need to consider more than small fixes. You may need to consider whether at this point you have not reached your human limitation, and grief-provoking as it is, Mom may now require placement.
If that is the case, then you may need to call EMS for trip to ER. At point of intake tell them you require social services help for placement as you cannot physically or mentally take your Mom back into your home. Do not argue. Do not swerve from that basic statement. Do not believe them when they deliver the platitudes of "We can work together and get you help" or "We can make this work". They cannot and they will not.
It is up to you how long you can sacrifice your entire life to this reality. I am so sorry, but that is the brutal fact. There is no fix it for many situations, and this is one for certain. I am assuming this deterioration is ongoing over time, not sudden as it can be in the case of a UTI. I am so sorry. I so wish you the best.