AI've read on other threads that the problem is the brand of powdered sugar I used. Unfortunately, it's too late to start over. Is there any quick fixes for this?

AUnfortunately, if you've gotten one of those weird bags of PS that makes your BC grainy there is no fixing it, at least to my knowledge and from what I remember seeing about it on here. I've had it happen to me before too, made me so angry!

ARun it through a stocking to get lumps out and try beating in some warm simple syrup.

AYou could also try to add some room temp butter to it to smooth it out. I had this happen to me and then a lady on here recommended that fix. It helped and I didn't have to throw it out