Constipation

My 1 year old has been straining to poop for months.

We talked to her doctor and they recommended mirlax for a few days.

We did that, it softened it but once we stopped giving it to her it was hard again.

We tried adding in pears,peaches,plums as recommended & it doesn’t seem to do anything.

could it be the whole milk? Can or should we be changing it? I don’t feel like I’m getting anywhere with the doctors.. I shouldn’t have to give mirlax every day of her life to settle her.

She sometimes pushes on and off all day, crying just to get nothing out.

She sometimes pushes for 20 minutes, crying and gets the smallest piece out snd tries again later.