If you've already told the interviewer in advance about your stutter and they reassure you it's fine but you still find yourself being rejected continously then perhaps it's best to personally reach out to them to ask for a bit of constructive feedback? Obviously not all interviewers are obliged to give each rejected candidate on how to improve as they're either too busy or unfortunately can't be bothered. It's natural to overthink in blaming yourself but most of the times it was nothing to do with your stutter but perhaps other candidates genuinely having a lot more working experiences and skills that amazed the interviewer, especially when candidates are biologically a lot older, so easier said than done, but don't take it personally!