I would definitely switch to MOM. I have heard from several people that colace can have an opposite effect on some, including myself. I t amde everything hard for me and impossible to go, I too, have suffered from impactions, that is what got me here in the first place, it was awful, to say the least!
I would really try the Milk of Magnesia, it is jsut about fool proof to get your stool soft, and easy to go. The trick is finding the right dose, my Dr. suggests starting with 2 Tblsp. at night bewfore bed, and working your way up from there.
I hate to sound so redundant, but I use something called Natural Calm, instead of MOM, it is a powdered form of Magnesium, totally natural, and works very well for me. You can look it up on the internet, and if you want any further info on it, feel free to send me a private message and I'll explain my experience with you.
Good luck!!