Slow Cooker Pot Roast- freezer update

I took my frozen bag of pot roast out and made it yesterday.  I don't think that I will try that again.  The carrots and celery were mushy, some of the potatoes were done and some weren't, and the meat didn't fall apart like it was supposed to.  Maybe I did something wrong, I'm not sure.  It cooked on low for 5 1/2 hours, then high for 3 hours.  I think next time I will just freeze the meat, then thaw it out all the way before using it in the slow cooker.