I’m not an overly successful, but I’ve done pretty well for myself in tech startups. I’m 27 and I’ve have/ had all the same fears you have. The one thing I’ve learned is that being scared of doing something makes my stuttering worse. Let your work speak for itself. Your stuttering won’t stop you from being great unless you let it. Being timid and not believing in yourself will stink worse than your stutter. I also find that I have a tough time in practice pitches, but adrenaline kicks in for the real thing and I usually do okay.