I try listening very carefully for the ę at the end of words (like myslę) and can't ever hear it. Does it get cut off in common speech?
If you are up for challenge, here is Polish radio audition about this:
In common speech it is often not pronunced or is pronunced weakly if ę is the last letter of a word. But it depends. I hear older people use more complete ę sound, in dziękuję, for example. They might go all the way with that.
Also, if it is in the middle of a word, it gets pronunced either as the usual way or "en/em" depending on what the next letter will be. Same goes for ą. ("ow" / "on/om").