Does Miralax Irritates the Gut?

I am so frustrated with IBS recurrences. As I try to manage the stress of NMI bladder cancer treatment, I am chronically constipated with hard pebbly stool. My bowels move daily, but movements are random, take a long time, and do not completely evacuate. It interferes with eating and sleeping. When Linzess was prescribed by a GI specialist, it was denied by my insurance provider. So, my PCP recommended taking Metamucil AND Miralax daily. They softened the stool. However, the cleansings continue to be painful. It takes me at least a week to recover from such flareups, during which I stop taking OTC remedies. Then, I have to repeat the process to avoid getting blocked. (Btw, I never have painful, nauseating movements with colonoscopy prep.) At this point, the bowel impaction--which shows on every CT scan--is causing chronic sinus inflammation and congestion. And I can always tell by my sinuses when I am getting blocked up again. I restarted Miralax at my doctor's recommended dosage to "get consistent." Nothing has changed after 3 days. I will try again, but I am afraid that the soft cleansing will irritate my gut again.