ANSWER:
You should make a lot of duaa for your cure, trust Allah and rely on Him. You should also practice for half an hour in front of the mirror everyday and this will improve your stammering with the grace of Allah and cure it.
You can make ruqyah over water or zamzam and drink/wash a lot from it until you are cured by the Will of Allah. You may need to sit in front of the mirror and practice speaking until you master doing this without any problem. Make Duaa in your sujood to Allah to remove this from you.
RUQYA FOR CURE:
Do ruqya on water, preferably zamzam and drink lots from it throughout the day and night and also bathe with this water until you’re cured by the will of Allah.
https://www.assimalhakeem.net/what-is-ruqyah-and-how-one-should-do-it/