i feel that they didnt deserve you and everything which happened is good for you and it must have taught you lot of things, so no need to worry at all. Apart from this analyse what made your stammering worse, was it low confidence or you were in hurry to deliver the answers or it was fear of stammering which eventually made stammering worse etc. Remember we need to identify our triggers and work on it specifically. My advice would be to give as my interviews you can and eventually you will get better, thats the rule of human mind and body in any aspect. Give it some time and keep on working on it. I am sure one day you will write a post on successfully cracking the interview and will be sharing your learnings.