Best way is give more presentations, until you get it right. Don't back off and feel depressed. Head on thru and face your fears. The first 3 times will be scary, sweaty and nerve raking. But i guaranty you that after 3 times your nervousness will be less then the first time. And after a while it would be normal to stand for a crowd and preform like you wanted to. Replace that bad memory for a good one and keep it that way. Practice makes perfect and every athlete practice their heart out to make that moment perfect. Own your self and you own the crowd. Yours, an "ex"-stammerer.