My MMF is crumbly and keeps breaking apart, I have tried adding shortening and also tried adding powered sugar. Neither of these worked. Any cures?

Warm sample in the microwave a few seconds at a time then knead it and see if that helps.

Warming in the microwave is a good trick.  You can also add a tablespoon of water at at time, not too much and knead it well.  Did you use a name brand marshmallow?  I found store brands to be a bit more dry and do not make a good MMF.  Good luck!