i know i am late but It's okay buddy you are not alone and you shouldn't give up because of some loser mocking you it's totally not your fault and it's just show what kind of human that team leader is. making fun of other's weakness even though he's a team leader. he's an idiot so just ignore his nonsense. i myself too struggle a lot with stammering i have been practicing for months but even then today i stammer when i was buying groceries i felt so embarrassed and disgust with myself but i am not going to give up just because of it and i know you can do it too. don't lose hope. all the best