You have a stutter and cannot change that. Focus on the things you CAN change. For me, that meant losing weight, eating healthier, and getting better sleep, to name a few. Put yourself in uncomfortable situations. Easier said than done, I know, but it will help. The 20th phone call you make will be easier than the 10th, which will be easier than the first. And finally, age makes a difference. I was very much in your shoes at 23, and I feel markedly more confident just at 28 now. You just give less of a fuck as you get older.