there comes a time when too much of a GOOD thing can become a bad thing....you need to have a backup plan so you can say " at this time, I believe this donation would be better suited for XXXXX" and just walk away...when it becomes WORK...it is not fun any more!
Another thing you can do is to PLAN what you are making...and only wash the fabrics in yardages that you need...say you need 3 yards for a backing...then only cut and wash 3 1/4 yards and let the rest GO!
Our group that sews for a children's hospice learned all this the hard way...the same way you are....now that we are losing the sewing room of the Bernina shop(she is closing), we can not accept anymore donations at all!