How do you change from a M4P file to something useable?

How do you change from a M4P file to something useable? All I get from iTunes is a message that says I can't change the format of copy protected files. M4P files won't play on my devices. If they only play on my computer they are essentially useless to me. I have a few years worth of music I've purchased that won't play off of my thumbdrive plugged in to my car stereo. I like to leave the stereo on random, so I didn't even realize till recently it wasn't even playing half of my music. So far the only way I've found to get around the copy protection thing that prevents me from changing the file to work, is to burn the music to disc and re import it. I don't want to have to burn and import hundreds of discs. How do I get this fixed?