Don't notice speech impediment.?

Apparently I have a speech impediment but I can't notice it. My words sound fine in my head but to everybody else it's difficult to interpret. I know I shouldn't care what other people think but it's kind of annoying when I have to repeat myself multiple times; and even then they sometimes won't get it. I'm 20 and am not sure which approach to clearing it up. I tried doing a voice practice thing my communications professor gave me but it didn't work too well.