I felt the exact same way growing up. It took a complete breakdown when I was 19 for me to be honest with myself that I was using stuttering as an excuse not to achieve. I knew I could do these things but I let myself skip them and put the blame on my victim status. It’s only been up from there tho. That perceived incompetence is why I pushed harder at work than my peers, and why I’ve made bigger progress than them. You kinda get an easy advantage in this regard too: other people will view you doing normal things as more impressive because of your stutter, so you can do the exact same work as someone else but look like you’re working harder. Anyway, it’s 100% a mental thing. I might stutter worse than ever today at 30. But everything is going great and I work in communications, so it hasn’t held me back.