There's a few options you have, I can think of one that I'd try.
What holds the rub onto the meat may or may not matter very much, I have had a great bark formation on using mustard, worcshesterchire sauce, or nothing at all, as a base. My bark forms in the last third or the cook, I find. And to keep it from becoming mushy, I don't tightly wrap it in foil to cool. I usually just put it on a aluminum cookie sheet, and drape it carefully to cool a bit. I find that this does help soften it up a bit, just enough to keep it from being too hard to chew and abrasive on the palate. By pulling and mixing, the textures and colors blend a bit.
If your bark is falling apart and softening to where its paste like, you might have one of a few things going on. Sugar, is very hydroscopic, meaning it will absorb quite a bit of moisture, if possible. Heavy applications of sugary rubs, that are tightly wrapped, allowing steaming meat to saturate the once tight crispy bark, turn it into mush.
Try less sugar, or not as much total rub, and dont tightly wrap.
And wait a bit for people wiser than me to jump in and contribute.
Bryan ? Kevin?